Reddit mentions of Complete Guide to Orchids (Ortho Books)

Northen's Home Orchid Growing is dated, but a very good book on cultivation. Bechtel's Manual of Cultivated Orchid Species is excellent- I think there's a newer edition out, but to heck if I can remember when it was published.

Ortho's orchid book is surprisingly good- mainly on cultivation.

Isobyl's New Encyclopedia of Orchids is excellent.

If she likes specific genera- paphiopedilums, phalaenopsis, cattleyas, dendrobiums, etc.- there are specific texts on each of these, and many more.