Dickfeld, G: 180 Tsumego

What is the most important skill in Go? It is the ability to calculate variations. In the middle game, about eighty percent is reading power, the rest is intuition. To improve your reading, it is essential to practice life-and-death problems. This book gives you 180 life-and-death problems with only one hint, whether the solution involves a ko or not. The solutions are limited to one diagram, so you may have to figure out on your own why your attempt to solve the problem has failed.