Don't know where to begin when preparing for the Power PE. Here are great resources to kick start your studying! In addition to @Zach Stone, P.E. course (I took the course while it was free), he has a great article which provides insight on each reference book. I brought to the exam a majority of his recommendations.
www.electricalpereview.com/recommended-references-resources-electrical-power-pe-exam/
www.electricalpereview.com/recommended-references-resources-electrical-power-pe-exam/