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 those books. Some categories contain subcategories that contain the links, for example the programming language main category contains lots of subcategories that contain all the various programming languages.
You have the option to switch the language and find books for that language, all major languages are supported.
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