I took the power module as well. First and foremost...the NCEES study guide. I thought that Machines, Drives, and Power Systems book by Wildi was a big help. I also had an old Engineering Economic Analysis book from college that helped me out on a couple problems. There were some topics I didn't have good references for such as those symmetrical delta/wye analysis problems, so I printed out a couple threads from this site...and I'm so glad I did. I also had bought that expensive MGE PE Readiness course (it has breadth/depth so there were only a few sections that still applied), but didn't actually do it, but there were a few pages that were really helpful in regards to motors.
PM me if you want additional info. I work as a consulting engineer, so a lot of the NEC, lighting stuff was really familiar.