Packt Publishing has a GREAT promotion going on right now to celebrate the release of their 2000th title. For a limited time, you can get two Ebooks for the price of one. You have an enormous library to choose from that covers everything about web, mobile, game, big data and cloud development.
What I really like about this publisher is that they are strong supporters of the open source community. They are not tied to one major vendor (like so many other publishers I know of). I think that makes them much less biased and open to promoting the latest technology and not just some big companies marketing agenda.
These guys are the ones that published the Visualforce Development Cookbook, which I posted about recently. I strongly suggest you take advantage of this limited time offer. I know that I am.