Found: Young Reader 90's Vengeful Ghost

I read a paperback novel sometime around 2003, from the local library--which makes me think it was published well before then. I believe it was part of a larger series that I never read. The plot centered around a kid (or two?) who went to a theatre and found two ghosts, a woman and man. It is revealed the female ghost was murdered by the man in a jealous rage. I think she was a star (vaudeville?) and rejected him romantically--my first experience with the trope 'if I can't have you, nobody can'. There was some kind of scene in a gazebo or shed outside for plot reveal.
I don't recall the cover, but the paper was that yellow-y pulpy kind in a lot of mass-market paperbacks of the era.

SOLVED: The Ghost in the Third Row by Bruce Coville.

Touchstone: The Ghost in the Third Row