The Chameleon Wore Chartreuse (2000)
A hardboiled detective. A young boy - vanished. A mysterious ... chameleon? This book was terrific reading aloud to my niece and nephew (7 and 10 years, respectively). They absolutely loved it (I could tell because they wouldn't let me stop reading that afternoon until we finished it!) Even despite their enthusiasm, I think I liked it more than they did.
Hale takes a tongue-in-cheek look at crime fiction, setting a disappearance at Chet Gecko's elementary school. The Chameleon Wore Chartreuse introduces the major characters in his Chet Gecko series. Read 'em all!