Les Miserables is the one I talked about earlier, wondering if it might be number 1. It's amazing. And I'm very sure that knowing the book just adds to it.
The musical does an amazing job, of condencing a brickwall of a novel into a three hour musical. But they left, obviously, a lot of details out. And some things simply hit so much harder, when you know those details. Like Fantine, one of my favourite characters, who you see in the musical only when she is already a mother. In the book, you get a chapter just about her youth, about her falling in love with a guy who then leaves here, after she becomes pregnant from him. Reading that part gives I Dreamed a Dream so much more power, because you know exactly what she is singing about. Also, the book starts with a chapter about the life of the bishop, which is such a nice introduction. It's a weird part, feeling like an episodic show about the adventures of this weird, lovable dude. It just adds a lot of texture, seeing more of him, instead of only having him show up for the relatively short (but still powerful) bit with Valjean.
There are such great bits. Just from One Day More, where Eponine sings about how much Marius doesn't really see her, while Marius and Cosette sing about losing each other, is so perfect for her situation.
The version that the video from One Day More is from has such an amazing Javert, too. He has an incredible presence. It has a weak Marius, and you hear and see it in the same video, where he simply can't compare in the duet part with Enjolras. But he does sing Empty Chairs at Empty Tables well enough, I think.
I dunno, I could keep on writing for such a long time. I could talk about Les Miserables for hours (and I already did, sometimes I meet someone who likes the musical, and then we can talk forever about it and the book). And I love it all, including the bits in the book about the canalization, life of women in cloisters or Waterloo. It's such a portrait of the time, and of specifically Paris, and just more then the story of Valjean who steals a loaf of bread, and is then forever hunted by Javert. That seems to be the general reductive summary of it, and it's just so much more.
Anyway, watch Les Misearbles. And read the book, if you have a lot of time. Both are worth it.
And don't watch the movie. It doesn't do most of the songs justice.