Elizabeth Prescott's website offers some fascinating articles about singing, on topics that are generally difficult to answer. Here are some of them:
What do I do with my hands?
Voice Teacher vs. Vocal Coach
How young is too young to study?
How long does it take?
What to take and not take when ill
But the most useful article I found was What they didn't tell you about your singing audition..., difficult advice, and useful for both musical theater and opera singers, who must brave much more rejection than acceptance before getting hired. Also take a look at breakdowns of several musicals for role and skill info.