Advanced Physical Chemistry

Here I thought that Intermediate Thermondynamics would always be my favorite in the series (because, well, Esther...), but Penny and Caleb are giving Esther and Jonathan a run for their money in that regard.
Following Penny on her journey from "good girl" to "fun Penny" made me smile a lot (and occasionally laugh out loud).
Caleb is one of the good guys, though I'll admit I got a bit frustrated at times with his "on" and "off" switch. I think a dual POV might have helped me understand him a bit better.

Bonus points for the supporting characters: especially George, who helped put certain things in perspective, but also Penny's knitting group (old friends from the previous books), and her BFF Olivia who always had Penny's back.