Hello,
I can't imagine that using this approach would impact on your umat score in a negative manner however critical thinking is only a part of one of the sections. Also in section 1 there is logical reasoning and problem solving and then the other two sections are a whole other story. Unless you are a natural at everything else (which some people are) then using this book alone will probably not be the smartest way to study for the umat if you really want to get into any of the courses requiring the umat.
As I'm currently preparing myself I know how it is bloody expensive for tutoring/company courses and unless you have your heart set on getting in to a competitive course it probably isn't worth it. I'm personally using the Medentry workshop/online resources (which once again is not cheap but I still got a group discount) and I have found that very useful so far and my exam score has already improved.
If you want to use the textbook approach though, if you're interested we were told a few books which would help with section 2, there are word banks online of difficult words used in the umat (because unless you know what words such as piqued, petulant, churlish, irascible and imprudent mean off the top of your head then you may need to work on it) and there are a few techniques for the section 3 questions which we were taught that I'm more than happy to share with you if shelling out the money to do a course/tutoring isn't a viable option.
Regardless of which way you go about it, best of luck for your preparation!