I am on my second time reading it.
I am doing it for the LSAT; however, I am seeing progress slowly, not as much as I am wanting to see. The book explains the different sorts of questions possible in the LSAT, and gets you comfortable with them. Not sure if the GMAT has a certain number of questions possible.
For the LSAT this is called one of the two bibles, so it is a necessary piece for self studiers. I would buy it, but it is really heavily focused on the LSAT, again I am not sure what types of LR is on the GMAT. For example, there are 13 types of LR questions that this book focuses on, and goes into details and example on how these types of questions might show up on the LSAT.
I haven't looked at a GMAT exam so I can't give a comparison answer, but if your strictly looking at increasing your LR skills, I do not think this book will do just that, but will help you recognize the 13 different types of questions.
The reason I say this book itself won't increase your LR skill is because for me, going through it was not the problem, but I didn't do enough practice problems to retain the information. I hope all that made sense.