You know what? Be prepared to have Christian Grey withdrawal symptoms after you read this book. I've been wandering for days looking for something similar, but couldn't find it. It's simply not there. I was recommended Gabriel's Inferno as a sort of the same book, but ended up DNF-ing it. It might have worked for me before Grey but not after.
Saying all that, book #2 is a bit weaker than book #1. There is a villain in the end whose actions I found highly improbable. People just don't do shit like that these days, and the whole deal reeked of early 90s contemporaries. However, Fifty Shades Darker is still brimming with deliciousness.
Ana is growing up at last and sorting out her priorities. Grey matures immensely but doesn't lose his sharp edge at all, and Mrs.Robinson gets what she deserves. Mwahaha!
The heat level is just as scorching, there are still those marvellous emails flying back and forth between the lovers, and January release of Fifty Shades Freed can't come soon enough.
Warning: do not read in public places. Believe me, it's for your own good. Side effects: up to 7 days of mild withdrawal symptoms, inability to concentrate and insomnia.