Wikibooks is a great service that collects books that are freely available. The bookshelf (aka the listing of all available categories) provides links to categories like computing (and its subcategories like Programming languages) and Humanities and in those categories descriptions and links to books hosted by the service. Some categories contain subcategories that contain the links, for example the programming language main category contains lots of subcategories that list available groups like C++ or Java where you find the books listed under.
You have the option to switch the language and find books for that language, all major languages are supported.
Update: Books are now offered in multiple languages. The English section offers more than 35,000 textbooks at the time of writing, followed by the German section with 16,000 and French with 9,000.
Each language section lists a number of topics, like languages, mathematics or the previously mentioned computing that lead to a listing of completed and featured books, as well as subsections if available.
You find many specialized books listed here, for instance about algorithms, the C Sharp programming language, or formal logic. Many books are not really introductions as a consequence, which means they are not suited for users who never came into contact with the topic that they cover before. Some books may have certain requirements, for instance another book on the Wikibooks page, that you need to read first before you can read it. Those requirements are listed on the book pages.
On the plus side of things, books are always available as pdf documents that can be downloaded to a local system for offline access and reading. Users who prefer printing can do that as well by printing a specially formatted version of the book optimized for printing. Note that books may not be completed yet which is indicated on category listing page and on the book's page.
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