The Writer's Handbook published by the Writing Center at the University of Wisconsin - Madison is one of the best writing handbooks online. It covers almost every aspect of writing in English and is highly recommended for all ESL and university students.
Features:
• Explanation of the stages of the writing process: planning to write, creating an argument, working with sources, drafting and revising your paper, finishing your paper
• Explanation of common types of writing: letters and application essays, literary analysis papers, proposals and dissertations, research papers, reviews, scientific reports, research projects and presentations
• Explanation of how to avoid common grammar and punctuation mistakes, how to write more clearly and persuasively, and how to cite references using different citation styles (APSA, APA, Chicago/Turabian, CBE, MLA)