Belle de Jour was the name of a famous blog that detailed her experiences as a call girl. She is an educated women, so there was no grubby. drugs or unhappiness. In fact it was all rather jolly. I read the book pretty fast, but I am not sure I found it that erotic. There was a plenty of sex in the book, most of it was pretty casual. I am not sure what the connection with the book was with Vienna, except that brothels are legal here.
There is also a great Bunuel film called Belle de Jour. I assume that is where she got the name from.
She is actually a biologist and was doing a PhD when she started becoming an escort. Anyway after reading the book I have decided not to seek out a job in the sex industry.