What your looking for looks a lot like the wikimedia project wikibooks.org.
Here a quote from the main page :
Welcome to Wikibooks,
the open-content textbooks collection that anyone can edit.
3,072 books with 83,999 pages
Also there are a lot of old textbooks in the public domain published in different languages. One repository of such books is project Gutenberg, where I find the category page helpful to get books in the right context/science.
Also some books, event recent ones are published with a open licence and as such you are allowed to read, copy and translate them at will, such books are for example published by framabooks (mostly in french).
These are must have books and there are some others to be found.
Also some book are freely readable online, copyright holder being aware, eg see VMLS if interested in maths or Motion Mountain for physics, but a big mouthful, not just 100 pages. (not sure for the licence, couldn't check it).
These are more often than not typeset in latex... So you might also ask on the author for the sources.
(I hope this answer isn't too much out of the context of the question)