It's incredibly heartwarming. Jake and Adrien are finally happy! You can tell simply by the fact that they laugh more, and they seem content just doing normal coupley stuff. 
 This book is also a mystery but there's just medium angst, which is perfect for a Christmas mystery. 
 Now our two favourite heroes are in a stable, "official" and established relationship, but they still have silly insecurities. For example, there's a good amount of jealousy on both sides. Well, I can't say no to jealous Jake and jealous Adrien. It's always entertaining. Adrien is as sarcastic as usual, and Jake is his usual protective and commanding - not that Adrien listens much. Headstrong is his middle name. 
 As usual in this series, there's just right amount of sex. Not too much, but hot and meaningful. There are also beautiful tender, awww moments. 
 We learn more about Kate. It shows that Jake still cares a lot about Kate and we learn that Kate is really going through a very difficult time, which is to be expected. . I'm happy JL didn't try to make Kate recover quickly or worse, suddenly become their "faghag" or something. 
 We finally get to meet Jake's family. This part is hugely spoiler material, so I'm just going to say, I've been been waiting for that for a long time. 
 I was a bit anxious, long-awaited sequels have this habit of disappointing me, but "So this is Christmas" made my day!!! 
 Two things before I end this review: 
 - Why is it said to be "Book 7"? I thought it was book 6? 
 - Adrien looks like (view spoiler)[Matt Bomer according to Jake's mum? (hide spoiler)]. No wonder Jake was knocked off his boots at first sight. 
 Nice way to start the year. Now if only we could have an Adrien English movie/series! Surely it's the next step!