All these books are free to read: either they're distributed with a free license, or they're in the public domain.
These books are not included in Project Gutenberg.
keyword: Software – 26 free books.
These books are available in EPUB format - good for iBooks, Nook, Kobo.
Aseem Sharma et al.
Sam Williams and Richard M. Stallman
Roland Rau et al. (editors)
Stefano Costa and Giovanni Pesce Luca