If you bought the reference book, buy this book. I know it's spendy, but do it. The 12th edition contains several great exam grade problems that anyone taking the PE exam should take time to work beforehand. I worked about 1/2 of the problems in this book, and felt well prepared for the exam. I was hoping the exam problems would be easier, but about half were about the same as the problems in this book (the other half were easier).
My frustrations were that the 12th edition still contains errors, and no errata was published when I bought this book, most were simple and straight forward, but some were frustrating.
The practice problems contain some problems that require references or ideas that are not contained in the reference book (see the problems in chapter 19, fluid power, and note that the book does not contain any methods for calculating hydraulic impedence)... there are only maybe 4 problems like this in the book, so it's not a big deal.
Also, I don't know why the author labeled many problems (1 hour limit)... some were very easy (like 2 minute problems), while other problems that weren't labeled (1 hour limit) took quite a while to compute. It'd be nicer if they were labeled (easy, med, hard) or something of that nature to make studying easier, so you could work all the problems in the area you want to emphasize in, while just doing the 'easy' ones in other areas.