You can start by calling Professional Publications to see what the current printing of this book is, particularly if you are using a borrowed copy. Because this book was designed in a certain way, there aren't too many options for how you can use it. When I designed it, here's what I had in mind. You should study every subject in this book. There is no longer any reason for a mechanical engineer to skip the electricity problems, or for an electrical engineer to skip the thermodynamics problems.
Difficulty is no longer an issue.
The only complicating issue is how fast you can work problems outside of your favorite subjects. Given enough time, you should be able to score 100 percent on the FE exam. You need to decide on a study schedule. There are 50 chapters and a sample exam in this book. So, you need at least 50 study days.