It looks like you’ve got a bunch of great suggestions but I’ll add one more. I just came out with a book of intermediate/advanced carols (mostly French ones) called “Light so Brilliant: Christmas Carols and Tunes for Solo Harp.” I wrote the arrangements for my Christmas album and I think they all make great solos – about 3 minutes each. The book is available through MelBay.com and probably elsewhere in a few weeks. You can listen to audio clips here: http://www.stephanieclaussen.com/album/light-so-brilliant/.