O'Reilly has published a number of Open Books--books with various forms of "open" copyright--over the years.
Related
O'Reilly's bookshelf: Reference Library
hell.org.ua · · Page
Free Online Books (mostly on smalltalk)
iam.unibe.ch · · Page
by Stéphane Ducasse
List of freely available programming books - Stack Overflow
stackoverflow.com · · Page
loads of programming ebooks
lab.lpicn.org · · Page
oreilly etc..
Game Programming Patterns
gameprogrammingpatterns.com · · Page
The C Book
publications.gbdirect.co.uk · · Page
This is the online version of The C Book, second edition by Mike Banahan, Declan Brady and Mark Doran, originally…