Okay, lemme try.
Merrily We Roll Along tells the story of Franklin Shepard and his friends, Charley Kringas and Mary. Franklin Shepard used to be a composer of Broadway musicals, but is now a jaded producer who makes lazy movies.
The show goes back in time, showing the development (and end) of Frank's friendships and relationships. It's something of a downer.
As far as sheet music, "The Singer's Musical Theatre Anthology" has a few songs - Soprano Volume 1 has "Not a Day Goes By," which Beth sings when Frank divorces her. "Now You Know" is in one of the Belter Volumes, 1 or 2, not sure, which Mary sings at the end of Act I. And Baritone/Bass Volume 2 has "Good Thing Going," which is sung by Charley.
The three leads are Frank, Charley, and Mary. Frank and Charley are both baritenors, I think, and Mary is a mezzo-belter. There are also three other, smaller principles, being Joe, a producer (I think he's a baritone), Gussie, Joe's wife who eventually marries Frank (a belter), and Beth, who is also married to Frank at some point (a soprano).
I would suggest looking at some songs from Company or Anyone Can Whistle for audition material.
More detailed plot synopsis and some song clips can be found here: