The Scheme Programming Language

The Scheme Programming Language - fourth edition

Available for free here
Or you can order the paperback version on Amazon: The Scheme Programming Language, 4th Edition

Use the Index Luke

Use The Index, Luke is a free guide to SQL performance for developers.

Unlike many other SQL performance tuning resources, Use The Index, Luke avoids unnecessary details about database internals but highlights the most important topic for fast SQL: proper indexing.

http://use-the-index-luke.com/

Clever Algorithms

The book "Clever Algorithms: Nature-Inspired Programming Recipes" by Jason Brownlee PhD describes 45 algorithms from the the field of Artificial Intelligence. All algorithm descriptions are complete and consistent to ensure that they are accessible, usable and understandable by a wide audience.

Get the book for free at it's official website: Clever Algorithms

Pro Git

Git is one of the fast evolving VCS.
Pro git is a free Git book that can be found here.
You can also support the author by buying the book at amazon.

Ruby on Rails Tutorial

Good Ruby on Rail tutorial available for both 2.3 and 3.0 versions.

Get the free version from here.

You can also support the author by buying the paper book here.

Invent Your Own Computer Games with Python

Great learning book for Python. You can learn the language by creating amusing games.

The book and the source code are available for free: Invent Your Own Computer Games with Python

If you want to support the author, you can also buy the book from here.

Little Brother

Little Brother is a book written by Boing Boing's author Cory Doctorow.

The book is available here: Little Brother in multiple formats.