I HIGHLY recommend "Understanding Space: An Introduction to
Astronautics"
by Jerry Jon Sellers. This book is very readable and gives an
introductory but very detailed explanation of all aspects of
spaceflight including propulsion, spacecraft systems,
guidance/navigation, and a very detailed explanation of orbital
mechanics. Some of the orbital material covered include the
calculation of the six classical orbital elements from two observation
vectors, patched conic section flight paths for planetary missions,
and re-entry calculations using ballistic coefficients. The math used
is at an advanced high school or college freshman level (very little,
if any, calculus, lots of vector and matrix math clearly explained in
the appendix). To get the most out of this book, you MUST work the
chapter exercises. I had to because this was a textbook used for my
Masters in Space Systems Operations Management ;^)

It's a little pricey at around $70 but it is 110% worth it if you want
a solid introduction to real spaceflight. This book will definitely
separate you as a real layman rocket scientist from the techno-peasant
astronaut wannabes.

An alternate less expensive orbital book is the classic (and somewhat
dated) "Fundamentals of Astrodynamics" by Roger Bate, Donald Mueller,
and Jerry White.

Good luck with your studies!

Martin