I had my nose in a book all the time as a child. My favorite fiction books tend to be science fiction, fantasy, mystery, or horse-related.
The Black Stallion by Walter Farley
Can you say "horse crazy"? The first time I was up on a horse, I was around 5 years old. It was so amazing to look down at the adults! I didn't live in the country, so I got my riding vicariously.
Arrows of the Queen, Arrows Flight, and Arrows Fall, by Mercedes Lackey
Take the idea of a horse and add in telepathic communication and brains: that's a Companion. Throw in a young, female protagonist who learns how to take care of herself in the face of adversity. This trilogy may appeal to the young (female) adult reader.
The Unexpected Mrs. Pollifax by Dorothy Gillman
A goofy mystery/adventure series with an older woman protagonist. No matter what happens, Mrs. Pollifax muddles through and comes out smelling like a rose. There is a movie based on the book, but I've never seen it. My cat Farrall is named after a character in one of the series.
Made for TV The Racing Game based on novels of Dick Francis
Dick Francis once was a jockey for the queen of England. His books often center around themes of horse racing and he describes the tracks in England so well, you'd think you were there.
Here's more about Mr. Francis.