[WildTurkey]

For light reading:

Nuclear Energy by Raymond L Murray, Pergamon Press, 4th Ed 1993
ISBN 0-08-042125-3
This book does oversimplify some things, but if you have no background in
physics or things nuclear, it's not bad at all.

For heavy reading:

Basic Nuclear Engineering by Foster and Wright, Allyn and Bacon, 4th Ed
1983, ISBN 0-205-07886-9

Introduction to Nuclear Engineering by Lamarsh, Addison Wesley, 2nd Ed
1983, ISBN 0-201-14200-7

A pair of my undergrad texts, both clear and concise.

For really heavy reading:

Nuclear Reactor Engineering by Glasstone and Sesonske, VNR Co., 1967,
ISBN not given. Difficult to find anyway.

[Wonder Warthog]

"Basic Nuclear Engineering by Foster and Wright, Allyn and Bacon, 4th Ed 1983, ISBN 0-205-07886-9"

Well, my copy is "only" the 2nd edition, but I doubt that the basics of nuclear fission have changed in the interim.