Arlene - I love what your association is doing! We may swipe the idea! That's so neat!!!
The Melody Bober "Hats Off To You" is a great duet book that is thematic. All the Melody Bober elementary music books are great in my opinion. She's def the best
Also, Vandall has some fun music books. As far as thematic... you're on your own. No suggestions from me. Sorry. I just am not creative that way!