Michael Kohl over at citizen428.blog() has a nice list of free programming ebooks. Many of these are for Lisp—including Emacs Lisp—but there’s a nice selection of other languages including Ruby, Javascript, Haskell, and many others. There are also books on miscellaneous subjects such as algorithms, parsing techniques, and functional programming.
I’ve seen this list mentioned a couple of times lately, but it offers a good selection of books so I’m passing it on in case any of my readers missed the other references.