Which Wikipedia sister project provides free textbooks and manuals?

The Wikipedia sister project that provides free textbooks and manuals is Wikibooks.

Wikibooks is a Wikimedia Foundation project launched in 2003 for collaboratively creating free digital textbooks, study guides, manuals, and other instructional books. Anyone can edit its pages, so a book can develop chapter by chapter rather than being published as a finished commercial volume.

The project began in English and later expanded into many language editions. Its subjects range from science and mathematics to programming, languages, cooking, and study resources for younger readers. Content is generally released under a free license, allowing reuse and adaptation under the license terms.

Wikibooks is easy to confuse with Wikiversity, but their purposes differ. Wikiversity focuses on learning activities, courses, and educational communities, while Wikibooks focuses on book-shaped resources. Wikisource is different again: it preserves or translates previously published free works, whereas Wikibooks is intended for original textbooks, manuals, and substantially developed instructional material.