Practical English Usage by Michael Swan was the book I used when I was learning English. I would like to have a similar book for German, since it contained a lot of common mistakes made by non-native speakers, as well as good explained grammar.
As for common mistakes, you might consider getting "1001 Pitfalls in German." It is out of print, but still easy to find. I own the variant for French, which covers topics like confused words, false friends, and other, well, pitfalls.