I thought that sheet music would be one of the last things to go Kindle (along with picture books) but Amazon just announced that they have just contracted to have a large library of sheet music available on the Kindle.
Assuming for size difficulties, the sheet music is recommended (“optimized”) for the Kindle DX, but it can presumably be read on the earlier devices. To see some of the titles available go to the Kindle Store and put in FreeHand Music and a composer. Bach’s Prelude and Fugue no.21 from Well-tempered Clavier is available here.
Lowest price for a score in the Kindle? $1.20.