By this, I’m assuming you’re a freshman in high school, yes?
He said he's enlisted, so I would assume older, in the military and just won't be able to work on pre-reqs/shadowing for a while.
But I agree there aren't really any books I'd recommend as far as giving you a leg up on preparing for vet school/pre-vet classes, reading veterinary textbooks would probably just be overwhelming. The most general primer on veterinary topics is the Merck Veterinary Manual, but it's better for a quick refresher when you're in vet school or out in practice, I briefly attempted to start reading the whole thing back when I was a pre-vet, but it's way too much.
(Also all the information is on their website anyway)
James Herriot's books are classic, or Tell Me Where It Hurts by Nick Trout is great. Just keep in mind if you haven't shadowed a veterinarian yet, books like this tend to focus on the most exciting and unusual cases, and don't really reflect the average day in the life of a vet (and obviously vet med has progressed a lot since James Herriot's day). An anatomy coloring book might be fun, DON'T try to actually learn the structures at this point, but just having seen the names before could be helpful, and coloring in general is a good de-stressor.
Here's an old thread on this topic with some more suggestions:
Books to read before applying/interviewing for vet school?
There's also the bookworms thread if you want more suggestions not related to vet med:
OT: Pre-Vet Bookworms