2.5 grumpy stars. Billionaire and poor nice girl trope. 
 Have I ever mentioned that I hate books with cliffhangers, particularly ones that simply end with a chop? Like coitus interruptus in written words. It is even worse when the book starts off reasonably well and gets better as the author begins - slowly, as I prefer - to develop the characters. I would read more ... if I were willing to play the author's / publisher's game. 
 Not dark erotic; the erotic content is mild and well done. This could easily be a 4 star book if it told a whole story and not just a third of it. 
 The heroine appealed to my male testosterone. She has a functioning brain and the willingness to take at least some action when the guy is not. The rich guy was too indecisive for me; women may fine him acceptable. (Apparently some do, as most readers of rich guy/poor woman books are women.)