Programming books online : Google Book Search
Google! Bless their little cotton socks! They are nice people (except any policy they have regarding China).
UPDATE: Go google! Way to stick to your guns (finally!)
They have a searchable collection of full and partial books online, which is great when you are trying to find book references on a specific code section.
Here is an example search for C# books that are partially and fully previewed.
http://books.google.co.uk/books?ei=Ea7cSOfFPILeyATv1Z3KDw&q=c%23&btnG=Search+Books
of particular note is this BRILLIANT almost-complete book by O’Reilly “Programming WCF Services”
