German AS /A2 - Zeitgeist: Grammar Workbook:

The quality of this book is not high, although it is certainly still useful as a means of getting practise with German grammar. The questions and answers contain mistakes more often than is really aceptable, and I think if you didn't have a teacher to help, this book might confuse you. For example, in some exercises, the answer booklet has more answers than there are actual questions, and in one of the tables for adjective endings, some of the endings are incorrect. This isn't very useful if you are relying on this book to be correct and to teach you grammar properly.

The book isn't that bad, and it's certainly better than nothing, but there must be better ones out there...