Jon Johnson also wrote a lovely, if brief, career respective bio on Cass back in the 80s. If I'm not mistaken, there's a link to email him on the "links" page of this site, and you can get the book directly from Jon. I ordered mine about 10 years ago from a local book store.
If you've read the liner notes to the Dream a Little Dream: The Cass Elliot Collection CD, those are also by Jon.
Anyway, it's a quick and fun / informative read. It also includes Cass's FBI file, which is great for a laugh.
Introduction by sis Leah, too.
Happy reading,
Kim