For a really well written textbook on astronomy, I would recommend Universe, by Freedman and Kauffmann.
Universe: Roger Freedman, Robert Geller, William J. Kaufmann: 9781464124921: Amazon.com: Books

For a more mathematical introduction to Astronomy as a subject, we have Astronomy - Principles and Practice, by Roy and Clarke.
Note that this book is quite heavy, content-wise.
Astronomy: Principles and Practice, Fourth Edition (PBK): A.E. Roy, D. Clarke: 9780750309172: Amazon.com: Books

You also need to have practice in sky observation. I'd suggest going to a local astronomy club for their star gazing sessions to get a basic idea of the constellations. You could also download a star chart.

Most importantly, you should make sure you understand the concepts of whatever you read and study. Questions, specially beyond the first level (NSEA) usually require knowledge of some physics and math, and the ability to apply it.

I wish you the best of luck in your preparations! Astronomy is a fascinating subject, and when you enjoy learning and reading about it, the sky is the lower limit!