Good Tactics Puzzles Books

I'm looking for some good books that are collections of Chess tactics problems.

Ideally, something that has different grades of difficulty and mixes up all the different tactical shots (forks, pins, skewers, etc.) without saying "the next 10 problems are forks".

I'd like to use it as a supplement to some de la Maza circles study.  I just find I can never do enough tactics problems ;-)

"The Ultimate Chess Puzzle Book" by John Emms is excellent and meets those criteria.

Sharpen Your Tactics has over 1000 beauties - truly some of the best tactics puzzles I've seen.  Actually, the notion that some tactics problems can be 'better' than others didn't even make sense to me before I started using this book.  :)

The problems are marked only by their level of difficulty, which trends upwards as you progress through the book.  There's no other guidance about the type of tactic or the specific objective, which is the way I like it.