YA/Late Middle Grade Equestrian Mystery - 1950s/1960s

Hi Everyone,
I'm desperate to find the title of this book I read as a kid, and which I loved:

Fiction, late middle grade/early YA.
Set on a ranch and involved a mystery around a missing/stolen horse (I think).
Main female character and two main male characters - a ranch hand named Red, and a mysterious newcomer named Sidney. There may have been a seance, but I can't remember now.
The writing style was remniscent of the 1960s/1970s.

Thank you so much

What year did you read it?

Was it part of a series like The secret of Shadow Ranch? Can't recall staff names.

>2 bookel: Thanks for your reply! It would have been in the early 2000s. Probably somewhere between 2004-2008! It had a sketchy illustrated cover (not the kind with a photo or really realistic picture of a horse) but I can't remember much more!

>3 bookel: Thanks for your reply! Yes, I think it was the first in a series, but my library didn't have any more copies.

Thanks for the info, I appreciate you.