Laura Lowe has written a concise yet definitive guide to what choral accompanists need to prepare in order to be a pianist for your typical volunteer church choir (TVCC for short). Part I looks at how to prepare the vocal lines and Part II covers learning the piano part, working with a conductor, and finding trouble spots before the first rehearsal. Hopefully a third part is in the works...
How To Be A Choral Accompanist, Part I
How To Be A Choral Accompanist, Part II