Ward Farnsworth, a Law professor at Boston University has released his two volume set of books on tactics online for free under a CC license. The books are available in an interactive format at Chesstactics.org. Hard copies of books are available through Lulu, a print on demand publisher, volume one and volume two
Before heading to law school I spent some (well… a lot of) time, playing and teaching chess. Portland had several great coffee house where you could hang out and play chess all day. Currently chess is a very closed market. Very few books or applications are under open licenses. Most of the top chess playing programs are closed source and both of the top database programs are closed. I personally spent a lot of money on Chessbase and Fritz. When I would teach children it became clear that the cost of books and programs was a barrier to helping some kids learn to play, especially if their parents did not have a background in chess.
It is refreshing to see a well written chess book online in an open format. Each section has a simple introduction and all tactical problems have solutions with explanatory text telling readers what is going on and why the solution is correct. Too many other chess tactic books provide only an answer with no explanation on why the moves are right. I recommend the Chesstactics.org website and the books for anyone interested in improving their chess tactics.
PS: Chess tactics followed by endgames are the most important thing for a new to intermediate player to study. Most games under master level are won or lost on tactical ability.