If you're planning on reading/listening to Mastering the Flames as a standalone, please don't. Not only did we meet Isaac and Constans in the previous books, but there's a lot of world-building and recurring characters.
The first time I read it, I felt the book started a bit slow, although I thought it was better the second time around. The flashbacks, which I hate with the fire of a thousand suns, didn't help matters even if they were needed, but after the 25% mark the book picked up the pace, and from then, I found it very difficult to put it down.
While there are several characters and a lot was happening, Ms. Himes managed to keep the main focus on this being Isaac and Constans' story. Perhaps how their relationship progressed was a tad fast, but they'd been flirting from the moment they met, so it wasn't that farfetched that they both wanted to be together. Plus, they had great chemistry and brought the best out of each other. Constans was all mysterious before, so it was great learning more about him, he was sweet and supportive to Isaac and seeing him stand up to Angel was a highpoint for me. I really felt for all Isaac had to go through as he worked through his guilt and his addiction. I wanted to hug him and make it all better and it made me so happy to see that Constans was willing to do whatever he needed to help him.
The whole thing with the blood mage was so interesting! While it wasn't that difficult to figure out who the bad guy was, it was still an engaging, action-packed case with many twists and turns that made for a very entertaining read.
Joel Leslie is just perfect for this kind of book and for this series, and it really added to the experience to listen to him narrate it. I enjoyed it so much, that I'll say that having read the book and listened to it, if you have a chance, just pick the audiobook. I promise you won't regret it.