These are websites and books that we recommend for further investigation.

YouthPLAYS.  The online publishing company created by the founders of ProduceaPlay.com, offering the best plays and musicals for young actors and audiences.  There is also a substantial collection of free monologues for teen actors (and others), a playwriting blog and more.

Stage Writers Handbook: A Complete Business Guide for Playwrights, Composers, Lyricists and Librettists
The very best book on the business of playwriting.
 

The Write Consultant.  Consulting for writers of plays and screenplays by ProduceaPlay.com co-founder Jonathan Dorf.

Actors Equity Assocation

The Dramatists Guild of America

Society of Stage Directors and Choreographers