In my opinion, I do not think we will ever get the true story behind a Geisha's life. If the inside-secrative realities were like the book claims, it would spoil the mystery and artistry of Geisha's outer image. I could see how keeping a secrative lifestyle secret would benifit and protect Geisha as well as the Japanese culture. If it were suddenly exposed that Geisha were "comfort women" for rich businessmen, it could be very damaging to culture and history of Geisha.
I think one of the problems is that geisha today are quite fundamentally different from the geisha in the past i.e. post 1958 and pre 1958 geisha.
Many geisha in the past started their lives in the flower and willow world after having been sold by their impoverished parents to okiya's/geisha houses. These girls became geisha because they had little other choice, some of their lives were quite miserable- for example, for as long as they were geisha they were not allowed to marry (they could have children, however many didn't due to the difficulties of raising a child in their environment), until they payed off their debts to their okiya they were also not allowed to choose who they wanted to be with (i.e. boyfriend/lover) freely, and if they weren't able to foster a good reputation they could expect no money/business and could only expect a lonely miserable and hard existence etc.
However they were of course many women who were happy being geisha- hardships for peasants in olden Japan were extremely harsh and the path to becoming a geisha offered girls a chance to rise above their dire poverty, a chance to afford expensive things like beautiful kimono and makeup, a chance to live a life without having to get married and a chance to eventually (if they were successful) have a decent amount of power over their own lives, work/business and destiny. It was a very strict life and the training was rigorous, hard and very long, but at times it was certainly easier to be a geisha than to be a peasant.
Its probably true that Mizuage/the selling of virginity did use to be practiced in the olden days, however although it wasn't technically mandatory, it does appear that many maiko (apprentice geisha) did do it because the potentially vast sums of money that could be made from it could seriously help the maiko pay off her debts and set her up for a more successful and independent career as a geisha.
And its true that many geisha probably did sleep with their patrons as a way of securing their patrons desire for her- a patron (i.e. guy who spends a great amount of money on his geisha in return for having a more intimate/personal part in her life) was vital to a geisha's success if she was to be able to afford the upkeep of her lavish lifestyle and remain popular and successful. But sleeping with a patron was never officially part of the deal and a geisha did not have to do it if she didn't want to.
Geisha were not prostitutes though- their appeal was in their beauty, intelligence and wit, as models, numerous talents like singing and dancing, and services like the service of banquets, social festivities and weddings etc.
Geisha today are fundamentally different in certain respects from the geisha of old because;
a. Geisha/maiko today are not sold into the profession by their parents.
b. They do not sell their virginity or any similar such thing.
c. Almost all maiko's today are in their mid to late teens when they start and are not children (because Japan made a law that makes girls attend school ("Parents are obliged to have their children whose ages are between six and fifteen receive six years of primary education and three years of secondary education at schools which are authorized by the government").
d. There is a very strict code of dress with maiko and geisha today (in an effort to preserve the image), while in the past the codes of dress were a lot more relaxed and experimental.
e. Geisha were cheaper to hang around with in the past- they have always been expensive, but due to their declined numbers (and the decline of many other professions closely connected to them) over the last century they have been forced to increase their prices to keep their business profitable.
etc.
The women who are geisha today are largely there in certain respects to keep the traditions alive (even though on the whole the profession is largely dying out), in some respects they are merely a facade/face of the geisha of the past. Japan's attempts to preserve their image and traditions/profession has ended up creating a geisha/maiko who is different from the ones that lived in the past.
Modern day geisha will almost always vehementally deny any connection in their profession to ancient practices like mizuage largely because despite it not been practiced for half a century now, geisha are still very misunderstood and the unfounded belief from foreigners that geisha are basically prostitutes has done their profession a great deal of damage and given many geisha a lot of unwanted attention from ignorant men. It might be true that it used to go on, but in their eye's there's nothing positive/understanding to be gained from admitting to stuff like that.
Its likely that Mizuage did used to go on in the olden days though as numerous ancient geisha have attested to this (like Teruha) and certain old documents also seem to agree. It doesn't really matter though either way as its long in the past now and is not relevant any longer to the lives of geisha today.
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