Back when I was still in the early stages of learning Kanji, I bought quite a few books and study tools to try to speed up the process. I never got anything out of flashcards, I think I only opened the box 2 or 3 times. Study type study books didn`t really hit home either. Children`s worksheets were wonderful... And one single book still hangs around for occasional reference. It is, in my opinion, the best Kanji study/reference book around.
Kanji & Kana
It`s great when you don`t know many Kanji, and you can still use it after you DO know most. All the other books seems to be very limited in the period of time you can make use of them.