Suitable for all ages, I think, though there are a couple of cuss words a few dozen chapters deep ...!
What is the character of the puzzles? Is there a variety?
Yes, there are a variety: logic problems, word puzzles, alphametics, various kinds of ciphers -- transposition, substitution, steganographic --, some coding challenges, and a whole slew of other puzzles that I don't quite know how to categorize.