Ohhhh West Side Story! I wouldn't have thought of that, but what a great answer :D

I think one of my favorite book-to-musical adaptations is The Secret Garden, which is a much more straightforward adaptation, but it's not completely straightforward. And then of course I grew up at just the right time for Les Misérables to be the most significant musical adaptation for my adolescent psyche, even if as an adult I don't consider it a particularly good one, musically speaking :)

I have not read either Lavinia or Black Ships. The latter is because I didn't know about it; the former I've meant to do since I read the Aeneid, and just haven't yet.