Eli Bendersky's website This novel is based on the author's careful study of the subject of geishas and many lengthy interviews with a real geisha. It is extremerly well written, which makes it a pleasure to read (and makes you finish it very fast) and is very interesting from a historical and cultural standpoint. Apart from providing a good understanding of geisha, the book presents many interesting facts about the life in Japan before, during and just after World War II.
I think that prior to reading this book I had a misconception of geishas, like many people I imagined them to be "fancy prostitutes". But it isn't like that - geisha in fact are well trained entertainers. They master many skills useful to help people "spend a good time" - dance, music, conversation, and so on. Sex is not what men usually summon geisha for. The usual occasion is some man throwing a party for his co-workers, taking them to a "tea-house" (akin to our pub, I guess), where some geisha entertain them with dancing, drinking games and chit chat.
Geisha actually do "sleep with customers" but only in a very special setting - marked by a traditional ceremony. When a man becomes a geisha's danna - he has physical access to her, but he pays a lot for that - he pays for the geisha's accomodation, her food, clothes, makeup and so on. In the narrative, this concept is rightfully compared to young women in the west that find themselves a wealthy grandpas (the most illustrious example is probably Anne Nicole Smith) to inherit money from, or just hook up with rich men craving for a higher social status, agreeing to be mistresses.