There is only a certain amount of development that you can make in German unless your grammar is fairly sound, particularly since the word order of German is very flexible, because it is the endings that tell you what the word is doing in the sentence. There are lots of grammar books in existence, and I think some are more suitable at certain times in your German learning career. So, in ascending order of difficulty:

  1. Essential Grammar of German, by Monika Reimann. Very clear in its presentation with lots of exercise that look a little bit tedious but actually aren’t and they are very helpful.
  2. Aktion Grammatik, by John Klapper and Trudi McMahon. Very well laid out, very well thought out and with useful exercises. As a level of detail the previous book doesn’t aim at.
  3. Essential German Grammar by Martin Durrell, Katrin Kohl and Goodwin Loftus. We find this is a good transitional grammar book for our first year students on the degree programmes (= courses).
  4. Hammer’s German Grammar and Usage, by Martin Durrell. This is most people’s preferred German reference grammar, as it is just extremely clear and well-organised. There is also the accompanying book of exercises, Using German Grammar.

Aside from that, there are countless online materials, though it’s difficult at times, unless you already know, to be certain of their reliability. On the whole, though, most of them are pretty decent.