I found the Open Source Books here on Google Rank 2.
OpenStax is part of Rice University, which is a 501(c)(3) nonprofit charitable corporation. As an educational initiative, it’s our mission to transform learning so that education works for every student. Through our partnerships with philanthropic foundations and our alliance with other educational resource companies, we’re breaking down the most common barriers to learning. Because we believe that everyone should and can have access to knowledge.
There is a great collection of books and I am simply in love with it. Before reading anything else now the objective is to master all these books.
You too can try your luck at OpenStax.org. The collection and the content is amazingly super natural.
There is also OER Commons on Internet which works similar.