Freely available books
Reference books are expensive and computer scientists often sit in front of their networked box, so what could be more obvious than reading online. Here we want to list links to publishers with freebooks or single titles.
Galileo Press Openbooks
Freely accessible HTML versions of selected books are available on the following website: http://www.galileocomputing.de/katalog/openbook/.
For example you can get the beloved "Java is also an island" there. Galileo Press is an independent German publisher of publications on computing, design and business topics. Galileo Computing offers titles on all major programming languages, tools and techniques: from C/C++ and Java to NET.
Wikibooks is an offshoot of Wikipedia and aims to create tutorial-like books online free for everyone.