It's a series of three books. Fantasy genre. Over 15 yrs old.

The first book opens with a girl (Cassidy) who finds herself riding a horse (Firefly) bareback with no idea of who she is or where she is. She's running from something or someone through a forested area.
Plot:
Cassidy is in search for answers as to who she is and where she is. Along the way she meets a friend who rides bareback with her on her horse Firefly. Her memories come back to her bit by bit as she continues on her journey. After meeting with an alchemist she discovers the world they are in is a type of in between place where people who are in comas, have autism, or other dysfunctions of the brain reside while their bodies are still existing in the real world. Cassidy remembers that she had been in a car accident with her truck and trailer which had her horse Firefly in it. Her actual body is in the hospital and she's in a coma.
There are a race of horse people who bond to horses by breathing into their nostrils when the are born. The King of these people is (Andy?) someone whom Cassidy knows in the real world. She remembers him being autistic(?) and riding her horse for therapy. However in the "in between" world it's as though he has an alter ego that is fully functioning while in the real world his mind is not all there.
That's the gist of it. I can remember much more so please ask if you need more details.

I did a few searches in WorldCat and also Googled a bit, but didn't come up with a title for you. Your best bet might be to call or visit your local library and ask the reader's advisory desk for help. They have print and electronic indexes to search by character name, setting, genre, etc. (Let me know if you find out what the title is.)

Thanks for looking! I'll try the library, good idea. :) If I ever get the title I'll be sure to let you know.

Librarians are cool! If you ever need to know something, think "library."