I think you'd have to go by author at one of the several ebook search engines.
Overdrive and
InkMesh are both ones mentioned on this site someplace. I think they're supposed to give you all the places that have the book and their prices, if I understood the threads about them correctly.
I accidentally ate some soy, so I'm partly incoherent right now.
Besides lurking here and other places to keep an eye out, if she's not super picky you could direct her to smashwords which has mysteries probably. I like smashwords for it's lovely multiple format presentation thingy myself.