First stop was Gould's Book Arcade in Newtown, a glorious warehouse-sized conglomeration of books, records, CDs and magazines. Only two of the hundred or so bookshelves was devoted to children's books, but I still found several titles to tempt me. The one I bought was the second volume of Hilary McKay's Casson series, Indigo's Star, which I hadn't succeeded in finding anywhere -- it's gone from our library, and this is the first time I've found it secondhand.
This filled in a few gaps for me -- I found out who Tom was, and why Rose loved him, where Indigo got his guitar, and more about the gradual disengagement of Bill from his family (which I must say his wife Eve seems to accept with almost supernatural placidity). Indigo is being bullied at school, while Tom has fled from his father's new family. Stars, music, high places and Rose's drawing combine for a bittersweet ending.
I've read the Casson books all out of order, and I'm not sure even now which ones I've missed, because some of them have similar names (Permanent Rose, Forever Rose, Caddy Ever After, Caddy's World) which is quite inconsiderate of Hilary McKay. Also they weren't even written in sequential order, which makes sorting out events even tougher!
Personally, I enjoy the warm, loving muddle of the Casson family. But Diana Wynne Jones wrote a rather scathing review of Permanent Rose where she classifies the family's dysfunction as more desperate, insecure and actually quite threatening to the well-being of the children. It's a fine line.