I've recommended these at other times, and though they're not strictly adult books, I have loved them, and count them among my favorites, as has my 12 year old son.

Sabriel, Lirael, and Abhorsen by Garth Nix. a fabulous trilogy.

I would second the mystery idea, Dorothy Sayers and Agatha Christie would be great summer reading.

I assume she's read Watership Down?

oh, almost forgot: Duncton Wood. the author's name escapes me at the moment... even better than Watership, I think....

formerly known as etaoin...