I was shopping a library sale and picked up several of these HorseDiaries books (for my Little Free Library). During my "MiddleGrade May" I figured I'd give a couple a read myself. They all seem to have high ratings ... and my 3* is perhaps a little more of an adult reflection. I think I would have liked these a lot in my younger years, and I would give them a thumbs up to any young reader. Very easy, anthropomorphized ... as the series title suggests, these are basically the horses thoughts, although not written in diary format (just a telling of their life).  There are 16+ books in the series, all stand-alones, set in various times/places. So readers are learning about history, as well as horses. They don't need to be read in order. Several authors/illustrators. 

I really hope children(and any adults reading) would continue to read the appendix after the story ends. Here, there are the author's notes, more about the history, and horse breed, given factually, giving the background on the fictional story. Good info!

No audio, but I was able to find a Kindle copy at my library in addition to the physical books I picked up. 

This book was my second read (even though it was earlier chronologically, I read #7 first). Oregon 1790. Appaloosa horse breed (Golden Sun is a boy, there is a girl horse that also gets quite a bit of story time). Native American Nez Perce (not sure I'd heard of these people before?) for the human interaction with the horses. 

These do feel "young" (as an adult reading) ... the human MC is 10/11 (I tend to view that as the aimed for audience).