Christmas Music Books
Bastien Christmas
It says "for Adults" (I guess because they don't use cutesy pictures inside), but I use this for everyone that's started into 8th notes. There are two very basic songs ("We Three Kings" doesn't even use 8th notes), quite a few in the Key of C, and several songs that are a good challenge for students in Levels 2A or 2B. The "Teacher Duets" are also quite manageable for a student in Level 3 or beyond and can be fun for an intermediate student to play along with a younger sibling.
Christmas Silhouettes
This has some beautiful arrangements that would be quite a challenge for students almost done with Level 2B, still a decent challenge for those in Level 3, and a quick victory for those in Level 4. None of these would make very good sing-along arrangements, but they are very pretty background music. If you want pretty arrangements that could definitely be sung with, or played as solos, see my Christmas arrangements in my music store. (I have several arrangements that do double duty.)
A Christmas Fantasy
These are somewhat more advanced arrangements (several of the same song titles), done by the same arranger as wrote Christmas Silhouettes. I don't use this as often as Christmas Silhouettes, but it is something of a step between that and A Christmas Offering.
A Christmas Offering, by Bethany Smith
Bethany is such a wonderful arranger! These songs are a very solid challenge for motivated students in Level 3B, still a challenge for students in Level 4 and 5, and advanced musicians will find interesting ideas with relatively easy victories. Her song "Bethlehem" is her original song that just "sounds like" the Christmas story! As a matter of fact, a careful reader can beautifully fit a reading of Luke 2:1-20, using this song as a background for the text.
An Offering, by Bethany Smith
These take some classic hymns and put some really neat twists on them - you can still tell what they are, but there are some really nice surprises in the arrangements. These songs are a very solid challenge for motivated students in Level 3B, still a challenge for students in Level 4 and 5, and advanced musicians will find interesting ideas with relatively easy victories. She loves to pull from the parallel minor, and even the relative major of the parallel minor! If you have no idea what that means, you can still completely enjoy these songs. But I'm hoping to post a free PDF pointing out some of the really neat ideas she uses and how they are fantastic applications of music theory!