The Composer is Dead by Lemony Snicket (with music by Nathaniel Stookey)
This is a fabulously fun book to introduce children to the orchestra. Written with Lemony's famously wry style, it made the Spozo and I laugh out loud a couple times as we listened to it (which you really should do as it is read by Mr. Snicket himself and the music makes it much more fun). The basic premise (as you might have surmised) is that the composer is dead and the Inspector must figure out who in the orchestra has committed this heinous crime. The music is modern Berlioz-ish sounding for much of it, but plays with some other styles too as they introduce the different instruments.
Two of my favorite parts are:
(One, from the very beginning of the book) "'Composer' is a word which here means 'a person who sits in a room, muttering and humming and figuring out what notes the orchestra is going to play.' This is called composing. But last night, the Composer was not muttering. He was not humming. He was not moving, or even breathing. This is called decomposing."
(Two, just a couple more pages in) "The violin section is divided into the First Violins, who have the trickier parts to play, and the Second Violins, who are more fun at parties."
The illustrations are also delightful. I highly recommend this book! (And thanks to Roni and Romgi for the awesome Christmas present!)