Today, I found out the good free resource for downloading ebooks. But I am not sure all links are alive or not, it is up to you.
You can browse other categories as well by using the homepage of this address: http://www.giuciao.com/books/
. Actually, that address collects a lot of books about Programming, Networking, Unix/Linux, …
Note: I am not responsible for those links. I just collect for you.