As good as the main book, I'd still recommend you get the verbal workout book as well. Sounds like this could really help. But like you mentioned, if you haven't gone through the main book, I'd do that first.
By the way, when you go through the questions, I'd highly suggest that you do it actively and not passively. In other words, as you go through the questions, do the following:
1) Do the questions
2) Check the answers. DON'T LOOK at the explanation.
3) Go back and try to figure out why you got it wrong. Make a note as to why you got it wrong. Is there a concept your not catching? Maybe a grammer rule etc...
4) Then if and only you feel like you really understood why a question was wrong, even if it takes 30 minutes to figure out, then go look at the explanations.
Looking at the answers explanation right away is a passive way to study. You end up just saying, "OK, got it" and then the learning is stunted.