Well, Ben Folds’ new album Lonely Avenue is a collaboration between him and Nick Hornby, who wrote all of the lyrics. I’ve listened to several of the songs so far and they’re quite good. I’m not sure if that’s exactly what you had in mind.
Odd that I just thought of the late, great Richard Farina, who died soon after having his first novel published, and also wrote and sang some great songs with his wife Mimi Baez (younger sister of Joan). A genius gone too soon.
Not just Hawkwind, but also Blue Öyster Cult (3 songs, and 2 of them are among the best BÖC songs recorded: Black Blade and Veteran Of The Psychic Wars (this song still gives me chills, and I’ve been listening to it for 30 years now)). He also had his own band back in the 1970s (Michael Moorcock and The Big Fix) where he sang and played guitar and mandolin.
Has Silverstein written stories in prose? The only Shel Silverstein books that I know are all poetry- not a far leap from lyrics.
Michael Nesmith has written two novels: The Long Sandy Hair of Neftoon Zamora and The America Gene
I read The Long Sandy Hair of Neftoon Zamora but not The America Gene, which is not available in traditional book form. From what I understand (and I haven’t delved too deeply) when you get the digital “book” there are several points throughout the story where you can choose on your own whether to follow links to pages of more detail on a particular point, or background information, or tangentally related supporting material- the idea that each reader creates their own reading experience. Sounds interesting, though I don’t know how accurate my understanding of it is, as I said, I haven’t checked it out yet.
Madonna, Kylie Minogue and Geri Halliwell have all written childrens books, but with them it’s very much a case of “Singers/lyricists who are also fiction authors, on the side, like once or twice, years ago” (although Madonna’s series eventually expanded to something like twelve books).
Nick Cave (of The Birthday Party, The Bad Seeds, and Grinderman) has written fiction, but this may again be a case of “lyricist writes books” rather than the other way around. However, these aren’t kids books, they’re full-on novels.