Shift is the sequel to Shade, which introduced a world where a Shift happened, and pre-Shifters can't see ghosts but post-Shifters can. Aura's the first Post-Shifter and her story sort of begins when her boyfriend, Logan, dies and basically, stirs up some shit. Zachary is a new love interest for her, and is the last Pre-Shifter and together with Aura, they can get up to some pretty cool/scary situations. 
 Shift excels as an sequel in that while it solves some questions that Shade brought up, it also introduced some new ones and readers are kept guessing as to what the answers are. There is progression within the story itself and it's good because it keeps things interesting. 
 In terms of character development, I don't like Aura's indecisiveness when it comes to her relationships with Zachary and Logan. One minute she wants to be with Logan but feels guilty about seeing Zachary, another minute she wants to be with Zachary but feels like she's betraying Logan as he's still technically on Earth. I actually thought Zachary's flare-up at the beginning of the novel was quite appropriate. A little harsh but undoubtedly true. 
 As for Logan, well, I can't exactly say I'm a fan of him even though I did like him before he died. I suppose my overall feelings toward him can be rounded up with a "Why won't Logan just pass on already?" While it is true that his character matures over the course of Shift, it still isn't enough to redeem him in my books. He's still the guy who does whatever he wants, whenever he likes it just because he can and it sorts of annoys me, as a character. Zachary too is put through some hardships as well and while I don't agree with all that he does, I still like him better than Logan. XD 
 Some light is shed on the mysteries of the Shift and the significance of Newgrange and without spoiling anything, I'd say that Shift definitely ups the ante and this series is definitely a series to watch out for. I am eagerly awaiting the conclusion to this fine trilogy.