One of the charms of The Diary of Tortov Roddle is the music by Kenji Kondo. It’s hard to pigeon-hole. Sometimes it reminds me of Metamora, sometimes of Satie, sometimes of something else I can’t quite place. Here’s a sampling of the soundtrack:
Kondo, I discovered, is part of the Kuricorder Quartet (formerly the Kuricorder Pops Orchestra) and is one of the musicians featured on the Azumanga Daioh and Yotsuba&! albums. He also plays Mozart on the ukelele. Here’s a video with the Kuricorder Quartet. I think Kondo is the one on the bicycle.
Addendum: Here’s the picture Author mentions in his comment. It’s from the booklet accompanying the first Yotsuba&! CD.