I second this recommendation. Allegri Miserere WOW! I understand that the early Roman Catholic Church wouldn't release sheet music for this piece (another good reason to go to church) so Mozart (yes W.A.) went to the Cathedral where it was being played, listened to it, went home and transcribed the whole thing from memory.

Even if it isn't true it makes a nice story.

BTW. I think the Tallis Scholars do a really nice version.