Oh--Oh!!
The mention of Watership Down made me remember this one (why I associate the two books in my head is beyond me, but anyway-- I do):
Jonothan Livingston Seagull. My dad gave it to me when I was about eight.
Transformative; the parallels to what life is like as a gifted child were unmistakable. It was as if the book was speaking specifically to me, about my life, and not just about seagulls.
I must have been in my twenties before I realized that
most people think it is some sort of new-agey parable about the meaning of life.

I just thought it was about the gifted experience.
One of my father's last gifts to me was a lovely signed edition he found for me (back when haunting bookstores was an art form) when I was about sixteen. It's one of my most treasured possessions.