Programming in Lua 2ed now sold by Feisty Duck (PDF only)
If you don't know about Lua, it's a very nice embeddable scripting language, with low memory consumption, very fast interpreter, and even faster just-in-time compiler. I loved it so much I added it to ModSecurity, and it is now possible to write rules in a proper programming language. It's great for those times when you have complex requirements. I am seeing Lua slowly but surely taking over the open source world (when embedding and fast and reliable operation is required). It's already in ModSecurity, Snort 3.x is using it, and in the future it will be part of Apache too.
The book itself is very good too, with a 5-star score in Amazon.com reviews.