Not sure if this is what you’re looking for but these are horse books that I actually enjoyed reading:
Beyond the Home Stretch by Lynn Reardon - Her experiences starting LOPE, an OTTB adoption program
My Horses, My Teachers by Alois Podhajsky - Well know for dressage but there’s nothing technical in it, just his experiences from a child through becoming head of the SRS. One of my favorite non-fiction books
The Horse Doctor is In by Brent Kelley - Reads almost like a veterinary guide except he gives real life stories from his experiences as a vet in Kentucky. So instead of just saying “here are the symptoms for pigeon fever” he also gives you an interesting case of pigeon fever he’s seen over the years.
Ruffian: Burning from the Start - Really good book but I couldn’t finish it.
The Faraway Horses by Buck Brannaman - lots of stories of troubled horses he’s seen over the years and how he developed his training philosophy
You could also look into the James Herriot books (All Creatures Great and Small). They’re not about horses exclusively but they come up pretty often in his stories.
I haven’t read the Seabiscuit book but I saw the 2 hour documentary on PBS’ American Experience based on the book that came out at least a year before the movie. It made my non-horsey husband tear up at the end. They have actual footage, pictures and interviews, including with the book’s author. The movie was pretty good but it pales in comparison to the real deal.