Increasing Chess Vision
Fred Reinfeld's book "1001 Winning Chess Sacrifices And Combinations" (http://www.amazon.com/Thousand-Winning-Chess-Sacrifices-Combinations/dp/0879801115/ref=pd_bxgy_b_img_b) is taking my calculation skill to the next level.
Its one thing to be able to calculate, but if the calculation vision lacks depth, clarity and comprehension, it will ultimately be ineffective. This book immediately shows me my "blind spots", challenging my vision with positions that look nothing like chess puzzles but are forced wins!
It is very instructive to be totally frustrated and then shown how I missed a continuation while actually trying to win with that exact variation!
Here's a fun sample that looks just like a "normal position":