You can't go wrong quoting Dr. Johnson to an English class:
Knowledge is of two kinds. We know a subject ourselves, or we know where we can find information upon it. When we enquire into any subject, the first thing we have to do is know what books have treated of it.
                        (before dinner at Mrs. Cambridge's house, 1775)
Johnson had no indexes, no card catalogs (electronic or otherwise), no online library information systems. There weren't any, and indeed there weren't any systematic bibliographies of any kind. We've seen that we can explore "what books have treated of it" in various ways, and we'll see more detail today. But the essence is not in the tools, rather in the curiosity of the searcher. One can make the choice to continue to educate oneself by following trails of interest, seeing where they lead, sniffing at all sorts of likely-looking items.
Some bits of general advice for electronic searching:
Last time we concentrated on using keywords to begin the process of searching, and emphasized the necessity to examine many candidates for relevance --both to develop a sense of the landscape(s) of discourse surrounding a subject (like 'gentilesse') AND to give serendipity space to operate.

This time we want to look in more detail at what an MLA search gets you, starting with a promising-looking result from our search for 'gentilesse':

TI: The 'Firste Stok' in Chaucer's Gentilesse: Barking up the Right Tree
AU: Allen,-Valerie
SO: Review-of-English-Studies:-A-Quarterly-Journal-of-English-Literature-
and-the-English-Language, Oxford OX2 6DP, England (RES). 
1989 Nov., 40:160, 531-537.
IS: ISSN 0034-6551
LA: English
PT: journal-article
PY: 1989
DE: English-literature; 1100-1499-Middle-English-period; 
Chaucer,-Geoffrey; Gentilesse-; poetry-; tree-imagery; relationship to heredity- UD: 8901
AN: 89-1-485
This article is a "Note" --a short article [7 pages] and not an "Article" or a "Review"-- and is mostly concerned with the imagery of a small part of the poem (the phrase 'firste stok'), its allusions and connections. In its relatively limited aims this 'Note' resembles the kind of analysis you will be doing on the text of the poem you choose. We are drawn into an interpretation of the text via an analysis of a key word.

A quick reading of Allen's Note (and you do have to read a lot of stuff --just collecting a citation isn't enough) tells us a lot that's of general use in developing a better understanding of the poem:

In his ballade entitled Gentilesse Chaucer uses an image of heredity to indicate a moral, as distinct from blood, relationship... opinions differ about the figure referred to by the phrase ['firste stok'] but all agree that _stok_ alludes to heredity. Literally meaning the trunk of a tree, _stok_ comes to represent the family tree... From its association with heraldry, _stok_ implies a rarified and exclusive line of descent. It stands in the poem in apposition not to a _fader_ of any social degree but to the progenitor of a morally aristocratic --or even royal-- line. The inference is that in being heir to the 'fader of gentilesse' one belongs to a strain much purer and more thoroughbred than do the mere offspring of courtly kings and aristocrats...
Allen also identifies Dante as "a certain source for this ballade" and traces lineage metaphors in other works. She goes on to discuss some structural features:
The ballade, with its rigorous and demanding structure, often results in semantic density just as the sonnet does in later English literature. The difficulty with the ballade's English career is not the structure of the stanzas (ababbcc) since Chaucer writes over a thousand in Troilus and Criseyde; rather it is the use of only three rhymes throughout the 21 or even 28 lines that poses such a problem. Given this exigency, it is inevitable that each word (line endings in particular) should relate closely with the others; that the necessary connection of sound should also connect the sense... For the skilled poet, the ballade offers the opportunity to display his virtuosity, his ability to produce a variety and density of meaning within narrow linguistic parameters. His decision to present blood lineage as the image of spiritual kinship is one calculated to compact the ballade's language into multiple meaning. The genealogical image operates as a metaphor, pun, paradox --as a conceit.
Allen uses 18 footnotes, many of which cite multiple references to books and articles upon which she has drawn in her research into 'Gentilesse'. Many of the citations point the way to significant sources that no amount of keyword searching would uncover, since they are on different subjects than 'Gentilesse', but (in the context of Allen's presentation) contribute to a developing understanding of the poem.

Arts and Humanities Search provides one means to explore the cited references: a search for 'gentilesse' in this database discovers the same article by Valerie Allen:

* * * * * * * * * * * * * Full Record Display * * * * * * * * * * * * * * *
DATABASE: A&H Search                    LIMITED TO:
SEARCH: su:gentilesse
 
 Record  1 of    2__________________________________________(Page  1 of  4)
|
|Genuine Art. No.:  CF837
|           Title:  THE FIRSTE-STOK IN CHAUCER 'GENTILESSE' - BARKING UP THE
|                   RIGHT TREE
|          Author:  ALLEN V
|       Doc. Type:  NOTE
|        Language:  ENGLISH
|            Year:  1989
|     No. of Ref.:  0023
|         Journal:  REVIEW OF ENGLISH STUDIES, V40, N160, 1989.  P531-537
| Journal Subject:  LITERATURE (PA)
| Cited Reference:  IN PRAISE OF MARY
|                   BOUTELL C, 1883, ENGLISH HERALDRY
|                   BRUSENDORFF A, 1925, CHAUCER TRADITION
|                   CHANCE J, 1986, CHAUCER REV, V20, P235
|                   CHAUCER G, GENTILESSE
|                   CUSSANS JE, 1882, HDB HERALDRY
|                   DANTE ALIGHIERI, CONVIVIO
|                   DAVIES RT, 1963, MEDIEVAL ENGLISH LYR
|                   FISHER JH, 1981, NEW PERSPECTIVES CHA, P177
|                   FRIEDMAN AB, 1958, MAE, V27, P95
|                   GARMONSWAY GN, 1972, ANGLOSAXON CHRONICLE
|                   KIKENHEIM L, 1963, GUIDE LITTERATURE FR
|                   MALORY T, QUEST FOR HOLY GRAIL
|                   PACE GB, 1982, VARIORUM CHAUCER
|                   ROBBINS RH, 1979, COMPANION CHAUCER ST, P380
|                   ROBBINS RH, 1980, INTERPRETATION MEDIE, P205
|                   SCGAN H, MORAL BALADE
|                   SCHLESS H, 1960, CRITICAL APPROACHES, P134
|                   SKEAT WW, 1897, S WORKS G CHAUCER CH
|                   STEVICK RD, 1966, MP, V64, P103
|                   STJOHNHOPE WH, 1953, GRAMMAR ENGLISH HERA
|                   WEISS A, 1985, CHAUCERS NATIVE HERI
|                   WILKINS N, 1969, 100 BALLADES RONDEAU
A search for one of these cited works reveals other writers who have also cited the same work (in this case Brusendorff 1925), and thus gives us some idea of the currency and influence of the cited work:
+ * * * * * * * * * * * * * List of Records * * * * * * * * * * * * * * * +
DATABASE: A&H Search                    LIMITED TO:
SEARCH: cr:brusendorff FOUND 51 Records
 
 ____NO.__TITLE__________________________________________AUTHOR_________YEAR_
|
|     1   THE REPUTATION AND CIRCULATION OF CHAUCER,...  BOFFEY J       1993
|     2   A NEW SOURCE FOR THE 'PAINS OF LOVE' IN RA...  BRAEKMAN M     1993
|     3   CANTERBURY PILGRIMS AND THEIR HORSES IN TH...  BOWDEN B       1992
|     4   A NEWLY IDENTIFIED MANUSCRIPT INSCRIBED BY...  GRIFFITHS J    1992
|     5   THE PENDANT IN THE CHAUCER PORTRAITS           BROSNAHAN L    1992
|     6   EARLY PRINTERS AND ENGLISH LYRICS - SOURCE...  BOFFEY J       1991
|     7   HOCCLEVE,THOMAS AND THE CHAUCER PORTRAIT       CARLSON DR     1991
|     8   CHAUCER SINGULAR PRAYER                        CRAMPTON GR    1990
|     9   THE SO-CALLED REPRESENTED PERCEPTION, A ST...  BRINTON L      1990
|    10   BRITISH-LIBRARY MS-HARLEY-78 AND THE MANUS...  LERER S        1990
|    11   HOCCLEVE 2 COPIES OF 'LERNE TO DYE' - IMPL...  BOWERS JM      1989
|    12   THE CHAUCER CANON - METHODOLOGICAL ASSUMPT...  BLAKE NF       1989
|    13   OF THIS COKES-TALE                             SEYMOUR MC     1990
|    14   THE FIRSTE-STOK IN CHAUCER 'GENTILESSE' - ...  ALLEN V      1989
|    15   REWRITING CHAUCER - 2 15TH-CENTURY READING...  LERER S        1988
|    16   ANIMADVERSIONS ON THE TEXT OF CHAUCER, 1988    FISHER JH      1988
...

Now let's take a closer look at what you can do with the MLA. Here are the various ways to search:
 __SEARCH______DESCRIPTION_______________________________EXAMPLES_______
|  
|  Subject     Type the label SU: and a word(s).         su:hamlet  
|              (Titles and descriptors)                  su:french literature  
|  
|  Author      Type the label AU: and the author         au:harner james  
|              name or any part of the name.             au:p g naiditch  
|  
|  Source      Type the label SO: and a journal          so:jornal de letras  
|              name, date, place, or publisher.          so:stuttgart  
|  
| A '+' following the name of a kind of search means you can search  
| simple plurals.  
|  
| KIND OF SEARCH          LABEL         REMARKS  
| Subject+                 su:          Includes titles and descriptors.  
|                                       The label is not required.  
|                                       EXAMPLES:  su:french literature  
| Author                   au:                or:  french literature  
| Title+                   ti:                     au:p g naiditch  
| Descriptor+              de:                     so:jornal de letras  
| Number                   no:  
| Series+                  se:  
| Standard number          sn:          ISSNs and ISBNs.  
| Source+                  so:  
| Update code              uc:  
|  
| BOUND-PHRASE SEARCHES  
| Author                   au=          A bound-phrase search looks for  
| Descriptor               de=          subject headings, author, etc.,  
| Language                 ln=          to appear EXACTLY as you type  
| Series                   se=          it.  We recommend that you find  
| Source                   so=          your term in the Wordlist first.  
|                                       Then follow the HINTS on that  
|                                       screen to search it.