Which are the best open books?

Working in academy taught me one huge concept about editorial world. Some publishing houses that work for profit tend to focus more on the quantity than on the quality.

For some months, I've been amazed at how many open books written by normal people (self-published resources) that need to be discovered. So I was wondering: is there anywhere a list of "open" books? Let's give some authors some notoriety.

A books/resource to be defined as open should be:

- a good quality content which is hard to achieve, especially when you do not have money to hire editors;

- free as PDF/HTML, but the author can still have donations, or can be paid for hardcopies;

- anyone can contribute to writing or editing! "source code"/content of the book is hosted in a Git instance;