In my opinion, yes, difinitive guide is good enough to get 90+. Another supplement to that would be John Rochfords book, which looks similar to the one he wrote for the Design and Technology course. I used both of these and got 93.
Rochfords book is really good for the Civil Structures Module, in particular the Ceramics section. It is a good read, although in my opinion, the only section of the course it falls down in is the truss analysis section.
So, yes Definitive Guide is good enough, it just depends on how much work you do on studying the textbook and the syllabus.