Published: August 1990
Publisher: Minstrel Books (1990-1996?)
Author: Unknown as of July 2021
Plot: In San Francisco, the Hardy's investigate a 3-year-old cold case involving a valuable race horse and how much money it's breeding rights would bring.
Review: This felt more like a Nancy Drew story!
The reason this felt more like a Nancy Drew story is that I've recently read the Files "Win, Place Or Die" book and that was about horse racing as well and the two books felt similar. Maybe Nightmare was originally a Nancy Drew book but the editors didn't want 2 similar Nancy's on the market at the same time or close together.
Anyway: 6.5/10
http://www.slate.com/blogs/nightlight/2016/08/11/a_nancy_drew_ghostwriter_on_the_art_of_being_carolyn_keene_and_the_evolution.html
Alice Leonhardt is possibly the ghostwriter. In the article, she mentions writing a Hardy Boys book about horses. The fact that the book felt more like a Nancy Drew book would make sense, as she wrote a bunch of Nancy Drew books.
She could have also have written Against All Odds or High Stakes.