These books are all on L1 writing, but if you are aware of any free online books on L2 writing, please let me know.
- Baldwin, Beth W. Conversations: Computer-mediated Dialogue, Multi-logue, and Learning
- Baldwin, Beth W., and Tim Flood. The Rhetorical Dimensions of Cyberspace
- Bazerman, Charles, et al. Reference Guide to Writing Across the Curriculum
- Brians, Paul. Common Errors in English Usage
- Brown, Kenneth, and David Barton. Brief Guide to Business Writing
- Burton, Gideon. Forest of Rhetoric
- Farbman, Evelyn. Sentence Sense
- Fulwiler, Toby, and Art Young. Language Connections: Writing and Reading Across the Curriculum
- Georges, T. M.. Analytical Writing for Science and Technology
- Hacker, Diana. Rules for Writers
- Harvey, Michael. The Nuts and Bolts of College Writing
- Horning, Alice, and Anne Becker (eds.) Revision: History, Theory, and Practice
- Langer, Judith A., and Arthur N. Applebee. How Writing Shapes Thinking: A Study of Teaching and Learning
- Into the Blogosphere: Rhetoric, Culture, and Community of Weblogs
- Lauer, Janice M. Invention in Rhetoric and Composition
- Lynch, Jack. Guide to Grammar and Style
- Lynch, Patrick, and Sarah Horton. Web Style Guide, 2nd Ed.
- Kies, Daniel. Modern English Grammar
- Krause, Steve. The Process of Research Writing
- Librivox, audio files of books
- Logie, John. Peers, Pirates, and Persuasion: Rhetoric in the Peer-to-Peer Debates
- McLeod, Susan H. Writing Program Administration
- McMurrey, David. Online Technical Writing Textbook
- NASA. Grammar, Punctuation, and Capitalization: A Technical Handbook for Writers and Editors
- Rhetoric and Composition, a WikiBook
- Strunk, William. The Elements of Style
- Young, Art. Teaching Writing Across the Curriculum
- Zemliansky, Pavel. Methods of Discovery: A Guide to Research Writing