Quick-take: Enjoyable middle book.
Gill and Solène part ways for some character-building as typical with a middle book. They bid goodbyes thinking they may never meet up again, but don't worry, we the readers know they will.
This is most definitely a middle book. It's sole purpose is to build up towards the 3rd book. Normally, there is a mini-climax to usher in the need for the final climax promised by the third book. I am not even sure this book ended with a climax. It was basically just more build-up and a cliffhanger. This book cannot be read standalone.
Fortunately, I don't really mind that. This book was entertaining, and I enjoyed the entire progression. However, a bit of a twist was added to the dragon lore that did not exist in the 1st book. I thought it was a very important factoid and simply cannot believe it was not known. In fact, much of Solène's revelations from her research seem too absurdly important to be lost to time.
If you can mentally accept very important dragon facts were long forgotten and accept a touch of rule-changing from book 1, then this book can be enjoyed. I do hope the finale is good as I pick up book 3.