I wasn't too crazy about Nicole's character and the cover image of her only reinforced that. The girl on the cover was IMO bleh. The continuation of the story did the same. It's odd how you can read so far along, entirely from her point of view, and yet you end up knowing very little about what made her special. Her personality was entirely ordinary. We get that she's smart, because she supposedly excelled at chemistry, but we don't know anything else as far as her school work, her aspirations, nothing. She didn't get along with her parents who mostly put their career over her. Ok but that still doesn't tell us a whole lot about her. She was nice and sympathetic -- ok that makes her kind of a Mary Sue. I have no idea why Jax fell so hard over her. For a story that sets a premise about universe defying pull of attraction, I feel like it could've used a heroine who was not so lacking in depth or personality. I think it would've worked better if she was someone equally extraordinary as Jax, as opposed to being "normal".
In contrast, the author continued to do a great job developing the main male character. You can really see the younger, more inexperience and tender person Jax, who was very unlike the older, more mature Cooper. His backstory was not particularly original, but handled well and very touching. The cover image also well chosen.
The sex scene was very very well done. Subtle and romantic and emotional.
Stylistic wise, eyes of various shades of blue/purple (and sometimes other colors) continued to perform all sorts of actions; they dance, they flip, they drift.[e.g. "Wild eyes turned to me.", "Light brown eyes watched my fingers..."] Not just eyes -- the constant use of overly colorful words to describe what characters were doing made it very distracting for me to read. ("When my hand flew up to wipe the chocolate...") I don't think I can read another story by this author if characters are always this animated. The writing would be easier to digest if the author doesn't try so hard to show vs tell. There comes a point where it makes the entire cast come off like they're in a circus.
I don't know about the imagery of "Even in your darkest hour, I'll be by your side." Probably it's just me, but every time that phrase came up, it reminded me of Frodo in Lothlorien: "May it be a light for you in dark places, when all other lights go out". Suddenly I was seeing Jax and thinking of a hobbit. So for me, that idea was not so effective because it came too close to one that had been used in a story as iconic as LOTR.
I was on the fence about buying the third book but wanted to find out what happened to Jax so went for it.