The Stefan Diaries are still the best line in the recent line of Vampire Diaries books, BUT.... it does feel like they're just rushing these out and chopping them down into shorter cliffhangers for no reason other than to sell more books and/or to get the books out fast while the show is popular.
This one picks up right where the last left off (and where the teaser had started in that book). The brothers Salvatore seeking out Samuel, Cora by their side. The plot is simple if you're reading in order (which you should be) -- they're determined to clear Damon's name, get Violet back, find out what Samuel has against them, tidy up all of the loose ends, and live as happily ever after as vampires can. They find their connection to Samuel (I won't spoil it). Beyond that? Well -- yup another cliffhanger.
Were they to just wait and put it all together in one version, I'd probably rate this better, but at this point it feels like they're dragging it for no reason. It's good as a single book, it's just not great, and not even as good as the earlier Stefan Diaries were as it doesn't feel like as complete of a story as those did.