This is the second book of teh trilogy that began on The Three Body Problem. And it is mindblowing. This is probably my favourite book this year, it is clever hard scifi that approaches the biggest questions about contact and proposes a "Cosmic Sociology", a science to understand other possible civilizations in the universe. And it actually makes sense!
I won't give it away, because it is just too much fun to to find it on the book, but if you have heard of Fermi's Paradox, and the idea that mathematically there should be lots of alien civilizations out there yet we seem to find none of them... well, the Dark Forest is a very good answer to those questions. This book is surprising fun, deep and even romantic in a weird way. You do need to read the first book of the series first, otherwise the Trisolarans and their whole sophon lockdown make no sense at all. But, trust me, it is worth it!
This is one of those books that stay in your mind and keep you wondering for days and weeks.
If you like scifi at all, you need to give this a read. (Also I hear a movie is coming up and all the cool kids will know the story before hand) :)