Two albums I really enjoy (I like hearing writers' demos anyway) are Sondheim Sings Vol. 1 and Sondheim Sings Vol. 2 which PS Classics released in 2005. Some of the material is very early and, while he's everything in the world but maybe not a top-class singer, hearing his interpretations gives a lot of insight.
I just came across them again recently (de-CDing my life) and was glad to revisit.
