Use the MERLIN and MOBIUS catalogs to locate books of criticism about an author or topic. 
Do a subject search on the author's name and scroll to an appropriate subheading. 
For example: Twain, Mark--Criticism and Interpretation.
Do a table of contents search to locate relevant chapters in books with a broader focus.
Note that tables of contents are not included in the catalog records for all books. Another way to find essays in collections is to use The Essay and General Literature Index (Ref. Alcove 9 AI3.E752 )
The keyword search searches for words in author, title, subject headings, tables of contents and notes all at once.  You can use various search options to make your search more precise and to sort the results in various ways.
You may expand your search to include all MOBIUS libraries. Materials requested through MOBIUS are usually delivered in about 48 hours.
If you haven't found enough, use WorldCat to search libraries throughout North America. You may request books from WorldCat by clicking on the button at the top left of each record.

Finding criticism in journal articles is usually a two step process. 
1) Use the following databases and indexes to see what articles have been published and where.  If you're looking for articles from a specific time period, be sure to use an index or database that covers publications from that time. 
2) If the full text of the article is not provided in the database, use the MERLIN catalog to see if we have the journal or newspaper and where we keep it. You may also check Find it @ MU to see if the journal's available electronically. 
If a journal isn't available locally, use the journal request form to tell us what you need. We'll order a photocopy from another source. This is a free service with delivery usually within one to two weeks. Feel free to ask a librarian for help.

 The major literary databases are: 
MLA International Bibliography of Books and Articles on the Modern Languages and Literatures
1963-present online, 1921-1962 in print in Ref. Alcove 9.
ABELL: Annual Bibliography of English Language and Literature
1920-1999 online.

Article1st
Web of Science
Expanded Academic and Periodical Abstracts both contain the full-text of selected journals. 
 

To locate older critical materials, use the following databases and reference tools.
19th Century Masterfile (1802-1906)
The American Periodicals Series Online (1740-1900)
The Wellesley Index to Victorian Periodicals 1824-1900 (Ref. computers)
The Reader's Guide to Periodical Literature (1890-) (Ref. Index Table 12)
International Index to Periodicals (1907-1965) (Ref. Alcove 2 AI3.R49)

The Literary Index indexes the many Gale literature criticism sets. These books excerpt literary criticism from all periods on individual authors.
Contemporary Literary Criticism (Ref. PN779.C6)
Twentieth-Century Literary Criticism (Ref. PN94.T88)
Nineteenth-Century Literature Criticism (Ref. PN761.N5)
Literature Criticism, from 1400-1800 (Ref. PN80.L57) 
Poetry Criticism (Ref. Alcove 9 PN1010 .P499)
Short Story Criticism (Ref. Alcove 9 PN3373 .S386)

Alternative and minority publications are not always well represented in the major databases. Use these indexes to hear these voices.
Alt-Press Watch
Alternative Press Index
Ethnic News Watch
GenderWatch
Index to Black Periodicals (Ref. AI3 .I552 -- Earlier volumes, including title changes, are in the circulating stacks.)
Women Studies Abstracts (Ref. Alcove 2 Z7962 .W65)

A Guide to Serial Bibliographies for Modern Literature (Ref. PN695.W67 1995) lists many additional specialized bibliographies.

Additional collections:
The Internet Public Library Online Text Page

Literary Resources on the Net

James L. Harner. Literary Research Guide: An Annotated Listing of Reference Sources in English Literary Studies (Ref. Alcove 9, Z2011.H34 2002)