Musicals 101
Musicals 101: How Broadway Musicals Are Made
It is a very comprehensive site on the Broadway musical. Though most of the site deals with Broadway, this particular page is very informative on how musicals are made. It is applicable for on OR off-broadway.
Other
Tony and Tina's Wedding
This is a long running off-broadway show. Though it is not classified as a "musical", it does have quite a bit of music and is worth investigating for it's uniqueness.