I just bought the sample exam from NCEES. It also comes with a CD-ROM,
and it costs about fifty bucks.
For the questions I couldn't answer (and for concepts I wasn't clear
on), I just used google. You would be absolutely amazed at the free
stuff that universities put out there.
Each question in the sample test is just that--a sample of what you
might be asked. Understand the general case and ancillary
concepts--remember the question you're answering is a single case.
Each question should prompt other questions in your mind. If not, take
a break; you're in a hurry.
As you investigate the general case and supporting ideas, keep notes as
if you were going to teach the subject in class the next day. The
exercise of making the notes and organizing the subject manner will be
much, much more useful than the reference manual.
Buying a ready-made reference manual AFTER rolling your own certainly
won't hurt. But, the process of making a reference manual for yourself
will be most instructive.
If you really want a reference manual:
Everybody else at the exam seemed to have John Camara's "Electrical
Engineering Reference Manual for the Electrical and Computer PE Exam".
Well, that and a suitcase, footlocker, or hand-cart containing about
seventy-five pounds of books.
(Amazon.com product link shortened)