DeLamater’s Christmas Music for Everyone is a perfect book for any occassion where you have a mix of instrumentation and need carols to play. It sounds simple, but this problem comes up a lot and there really isn’t a lot of material out there for this sort of situation.
The way it works is that each two page spread has two tunes on it, with parts for C instruments, Bb instruments, Eb Instruments, and Bass Clef instruments. All you need to do is turn to the page, and everyone just reads the part that suits their horn, and you are off to the races. On top of all of that, it includes a piano score with words, so you can have a sing along and play with an accompanist all at the same time.
The samples show what a two page spread is like and show the score as well. Check it out, you are gonna love it!