Manning[1], the Pragmatic Bookshelf[2], and Apress[3] have been the best in my experience. They cover relevant topics and technologies, the books are typically written by experts and written well, there are ebook versions of everything at reasonable-ish pricing, and sometimes you can get early access to books before their finished.

1: http://manning.com/ 2: http://pragprog.com/titles 3: http://www.apress.com/