Quite a while ago I bought the book Why?: Powerful Answers and Practical Reasons for Living LDS Standards by John Hilton III and Anthony Sweat for my children and they have enjoyed reading it. It addresses 29 broad questions, broken down into more specific questions in the 200 pages of answers. The great thing about it is that each question has very short articles or facts with lots of photos and graphics. The answers are doctrinal, funny or statistical.
The format is easy for kids because they can just pick up the book and start reading wherever, even if it's only for a few minutes, and they can still get something out of it. I'm sure we'll be using this book for years to come.
Examples of some of the questions are:
*Why shouldn't I date until I'm 16?
*Why can't I listen to a song that only has a couple of swear words if I hear worse than that every day at school?
*Why do some people who pay tithing still not have a lot of money?
*Why should I listen to my parents even if I think they're wrong?
*Why should I repent now and not later?
They answer questions youth might have about standards found in the LDS For the Strength of Youth pamphlet. What youth doesn't have at least a few questions about them?