The Modern Language Association style is most commonly used in the humanities. The latest version is 8, which came out in April 2016. Check with your instructor about which version they would prefer that you use. Note: many of the library databases' built-in citation tools are now using MLA version 8.
MLA Handbook 8th Edition MH Reference LB2369 .G53 2016
MLA Style Center The MLA's own resources explaining the citation style. Includes a Quick Guide and FAQ.
MLA Citations From Fairfield University DiMenna-Nyselius Library. Nice explanation of the changes from MLA 7 to 8, plus well arranged examples of commonly cited types of sources.
MLA Formatting and Style Guide From Purdue University Online Writing Center (OWL)
Understanding MLA Style (8th edition, 2016 updates) Quick video explanation of the new MLA 8 format.
MLA Handbook for Writers of Research Papers, 7th Edition MH Reference LB2369 .G53 2009
Diana Hacker's Research & Documentation Online Includes instructions for how to document sources in MLA 7 (Humanities), APA (Social Sciences), Chicago (History), and CSE (Sciences) styles. For examples of how to cite different types of sources in MLA 7 format, see especially the MLA list of works cited page.