This site lists free online computer science, engineering and programming books, textbooks and lecture notes, all of which are legally and freely available.

Very well categorized. Equipped with advanced search engines.

SaltTiger | 每天一本编程书，每天进步一点点

鸠摩搜索引擎

Easy and fast book searcher, create and search your private library.

中文数字图书馆,图书来自Z-Library.