Nice.
Here is another good site (instead of downloading pdf:s, you can browse the books/chapters). https://www.programming-books.io/
It seems like both sites are based on material from the Stack Overflow Documentation project, now discontinued.
As a matter of fact, some chapters, for example in the Android book, are not up-to-date.
Nonetheless, it is a goldmine with a lot of useful information which are still valid.