Maybe she's hooked on the Internet.
When my husband and I started our relationship, he had just bought the house we still live in. It contained some built-in bookcases which he had already filled. So he built me a wonderful floor-to-ceiling bookcase (I confessed I always had an ambition to have one). It didn't take me long to get that stuffed to the gills, and we have each since bought more bookcases. Our problem now is that we don't have any more floorspace for more bookshelves, so e-readers came along in the nick of time.
We do have clean-outs from time to time and donate the results to our favourite charity shop (RSPCA) but mostly we can't bear to part with our books, just in case we ever get the time to reread them.
We do have other interests in common, and some we pursue separately, but I couldn't imagine living with someone who didn't love books and animals.