There isn't anything specific as far as I know. A few wildlife books might mention bunnies but not in great detail.
How about something about identifying hedgerow plants and their different medicinal uses? Or something on drying plants? You can feed them dry which would let her collect things and store for winter.
You can also buy seeds for some common wild plants which can be grown in pots to feed to bunnies. I've can a couple of seed trays full I lift in/out to let the rabbits much on.
Some of the wild plants are the best things for bunnies with delicate tummies so she could be on to something