Hattie Breaks a Leg (Theatreland Mystery #3) by Patrick Gleeson

Hattie’s down on her luck. She’s collected enough enemies in high places that the only work she can find is a one-night-only vanity project being mounted by an extremely unlikely impresario, and beyond that things are looking bleak. However her career concerns must be put aside when an old friend appears, in serious trouble and asking for a big favour. Hattie tries to help, but things swiftly go awry and she finds herself thrown into a cat-and-mouse game with some very shady types who pursue her across London all the way to (where else?) backstage in the theatre on opening night.