Anyone know any books on writing musical theater/musical theater songs?
I don't know any books but I recommend watching Leonard Bernstein's The Unanswered Question and Omnibus DVD, you can find them both of youtube.
I don't know of any but I'm sure they probably are some. I, however, think your best bet would be to just look at existing scores to see what others have done. Then you can figure out the trends for different styles and figure out how you want to do yours.
I would just invest in some scores from musicals and some DVDs. or go to some. I've played enough to have a basic idea of how to write one. the most important thing is to have a few really good motifs to reuse throughout. oh, and arbitrary key changes every six measures.