I disagree on Les Miz. (Don't hate me for this Stick)
But, Les Miz is a preference read. Honestly, it's almost nothing like the musical, and the only thing they don't mention in the musical that pertains to the main characters is that Gavroche is Thenardier's son (Oops, was that a spoiler?) Yes, Eponine and Gavroche are brother and sister. There, now that I've said it, you no longer need to read the book. The truth is, Les Miz is a hard read. Much like the Illiad, there is so much irrelevance going on that it's hard to tell what's what and where's where. All the basics are covered in the musical. The rest of the stuff was left out because, well, it's pointless to begin with.
Other books turned to musicals:
Aspects of Love (actually, almost anything Webber put his name to.)
Dessa Rose
Once on This Island (Titled "My Love, My Love or The Peasant Girl" by Rosa Guy, which I HIGHLY recommend)
"Do you know what pledge time is, Andrew"? said the PBS Executive.
"Yes", Lloyd Webber replied. "My 50th birthday special must be one program that gets done a lot."
"No", mused the man from PBS heedlessy. "Not so much. Our Stephen Sondheim Carnegie Hall concert. That's a big one."
Spoons, forks and knives seemed suddenly to suspend their motion in horror, all around the table.