We would be remiss if we did not mention the women whom I consider to have been the goddess of all ballet accompanists, Harriet Cavalli. She has a book out on playing for the ballet and is available from either the University's web site or hers:
Her CDs are quite good too and will give you a good flavor of HOW she played -- what I call a "full-orchestral sound." The energy of how she played was always lively and fully supportive of the students and dancers.
One of my personal favorites is the dotted 6/8 march ("El Capitan"/Sousa) she has on the second CD listed -- "Music for Ballet Class 2." Ditto with many of the tracks on her Perry Brunson tribute.
As a long-time (and sometimes long-suffering) ballet teacher, can I offer my short (I promise!) advice. Stuff that seems obvious but...
In priority order:
1) correct meter
2) feeling that matches exercise (sometimes, for me, this is the most important criteria!)
3) length of music and exercise come out the same
4) good intro: two or four bars with a very brief pause (about a short breath) between intro and start of tune
1) lots of octaves in the left hand/bass clef
1) pliés are not a weak exercise, even though the dances are relatively still -- please no tinkle-box music or stuff that's "atmospheric" give us some inspiration: lively, energetic.
2) music for the stretch part of rond de jambe par terre should speed up slightly into a bouncy 3/4 -- not slow down or be "heavy." This comes after the fourth phrase of 8 (usually).
3) play arpeggios that go UP at the end of exercises and "amens" after balances to get us down.
4) throw in a occasional show tune
While it's true that there are fabulous accompanists out there who have not ever stepped into a ballet class, it DOES help! If anything, to feel how the movement should go. A friend that went to my high school and who later played for SFB School, reported to me that when she played for ballet classes at the UW, they had her "run around the studio" when she was beginning.