Originally posted by emperor31
I can't help you sort by difficulty, but don't see why anyone would need to do that as you've listed the two best books on tactics IMHO:
"Chess Tactics" by Littlewood and "Understanding Chess Tactics." Both are gems and will repay dedicated study.
Course in Chess Tactics by Dejan Bojkov and Vladimir Georgiev is awesome, but I'd hold off until you've gone through the other two multiple times. The Dreev/Seirawan game is stunning.
The reason why I would like to have them sorted by difficulty is to set order for studies.
For example, which of the books that you have recommended as gems should be studied first (is easier)?
"Chess Tactics" by Littlewood or "Understanding Chess Tactics?
(I would say that easier is the book which contains combinations of lower depth (plys) on average.)