A few things:
Besides "First Impressions," isn't there a British musical version of "Pride and Prejudice" simply called that by Bernard Taylor? Does anyone know if it's any good?
There's supposedly an opera version of "Confederacy of Dunces." John Kennedy Toole's mother would sing selections from it when she went on the lecture circuit.
As for the "North and South" musical, I have three words: "Shogun: the Musical"
As for me, in the spirit of "Oliver!" And "Copperfeld!", I've always wanted to do a musical version of Charles Dickens' "Bleak House" and call it "Bleak!"
But seriously, I tried to adapt a terrific novel called "The Ballad of Typhoid Mary". I gave up when I realized that it was turning into "Sweeney Todd" in drag.
I think that anything can be made a good musical, if the people doing it are talented. Look at "My Fair Lady," before hat no one thought that you could adapt Shaw, then "Lady" opened. Then there were several Shaw musical and none of them were good.
Though, even if the writers have talent, it's not a guarantee that the show will be a hit ("Lolita, My Love," "The Grass Harp," etc.)
"A coherent existance after so many years of muddle" - Desiree' Armfelt, A Little Night Music
"Life keeps happening everyday, Say Yes" - 70, Girls, 70
"Life is what you do while you're waiting to die" - Zorba