I have read target band 7 and some pages of Ace the IELTS. It seems to me very good book. Though tt can not completely transform you to get band 7+ if you do not have a good base but definitaly can help you out in achieving good score if you can understand/speak at moderate level. In short if I have to buy any book I will buy it.  

Review: Ace the IELTS: IELTS General Module - How to Maximize Your Score

A good short book about the best strategies to get a good score in the IELTS General Test ,though I didn't get the point of writing 2 books on the two modules of the IELTS with almost the same content ... Read full review