I remember this book always being on the shelf in my grade-school library. I never read it, because a glance at the cover always made me think, "Clearly this is about seafaring rabbits," and I don't know why my fantasy-loving self thought me too good for seafaring rabbits at the time, but at any rate I never read it.
Two things: It is not about seafaring rabbits, and it is awesome.
The cover already proclaims this thing a classic, so any praise of mine is superfluous. It's an adventure story--gave me the same feeling as my memories of reading Swiss Family Robinson as a child--and the world is one I thoroughly believed in. It is about rabbits.
Incorporated into the narrative are rabbit myths, which made me extremely happy. Myths or fairy tales are one of my favorite things to make up, and so it was encouraging to see that they can be used, and used well, in a modern book. (Well, relatively modern--it was published in the 70s.)
I borrowed this from a friend and intend to get a copy of my own posthaste.
Age I'd let Z read it: Whenever she wants. 8 or 9, probably.