Cho U's Tsumego
Cho U's Tsumego (張栩の詰碁 Chō U no Tsumego) is a very difficult tsumego book with a novel approach written by Cho U. The hardest problems have subproblems, which consist of refuting a single possible line of play. The subproblems are considerably easier.
The book, published in 2006, by Mycom (Mainichi Communications) carries the ISBN: 4-8399-2124-5.
A review of this book is available on New In Go at the GoGoD website.