I'm such a huge fan of this author's writing. It's quirky and funny and just so very British. One Under is a prime example for that. It's written alternatively from Mal and Jory's POV which gives you great insight into both character's feelings and back story. 
I was a big fan of Mal in particular. He could be quite frustrating but he had reason to be because he's just getting over a traumatic event that turned his life upside down. I loved how Jory dealt with everything Mal dished out which quite frankly was a lot. Still, their dynamic was amazing.
The only reason this wasn't a full 5 stars for me is one particular mishap that happened quite late in the book. And that mishap was quite the whopper and I felt it should have been talked through more thoroughly/clearly. 

But even without that I loved One Under and I really hope the author will write more books for this series.