I read RK's book long ago, also Forsyth's, Corzine's and Sebesky's (the best so far for this topic) but they are either outdated or skew to recording or huge orchestras. As of the last Broadway musician's contract (following the strike) the largest houses have a minimum of 18 pit musicians (19 if you count the conducter). Some are down to as few as 5 for a first run Broadway show. For those of you who are not connected with the theater, producers normally do not want to pay for a single musician above the minimum. It's nearly impossible to revive any of the classic musicals with their original orchestrations. I recently read an interview (in Playbill, I think) where a contemporary orchestrator said that 20 years ago he remembers having 15 violin players in the pit. These days, most shows have 15 or less players for the entire orchestra. (And don't get me started on how much this number is reduced for tours and other venues without Broadway's capital.)
I have been a professional actor for more than 20 years. I have occassionally appeared in musicals and I have dabbled in writing musical theater. The last time I tired it, my score was hurt by a poor orchestrator. Now that it is possible to produce a reasonably good demo on a personal computer with sample libraries, I have spent the last couple of years learning how to do this, buy all the necessary equipment and teach myself how to orchestrate, so that my music will be presented at its best. I know full well that, if it ever goes to Broadway, it will be orchestrated by one of the few people who do it all the time -- and it will sound great, even with the smallest pit -- but, realistically, this has to sound good to smaller companies, who will most likely want a print out of the score, right from MIDI tracks in Sonar (score view).
There are no good books on orchestrating for the musical theater. At any rate, none that deal with the realities of the situation today. You have to make it sound BIG with few instruments. You have to be able to suggest a lot of different genres (often in the same show). Most orchestrators cop out and replace the string section with sampled strings. Audiences can always tell. The comment I hear most often was that they thought the show was recorded or partially recorded.
But, it's more than just that. A lot of musicians double on several instruments (especially the reed players). You need to know how to realistically double, when you can get away with close harmonies. I know there is a "Broadway" sound (jazzy, brassy, big-bandy) but it's not right for every show -- and not that common any more. You need to be able to suggest any style and have it sound full when you want it to be full. Even the choice of instruments (with so few players) is a big question where some guidelines from the pros would be helpful.
There's a web site (http://www.custombroadwaymidi.com/mp3_samples.htm). This is for a company that just takes the Broadway scores and sells MIDI files that they create from them (for production companies that have NO budget for an orchestra). This is not meant as an endorsement or a condemnation of this company. I have never dealt with them before. I just found their site and started listening to the samples, because I wanted to hear some Broadway tunes with just the accompaniment. I was struck by how much variety there is in a single, Broadway number. The audience's attention is never allowed to waiver for a second. THAT's what I want to learn how to do.
I corresponded with Vince Corzine for a while. He was planning to do a book about this, but the last I heard it's been put off indefinitely. What would really help me -- and anyone else interested in how to right for relatively small groups of musicians to perform live -- is a book or online seminar that deals with this subject.