This book felt like it took forever for me to finish. I read the first 1/3 pretty quickly, and that the second 1/3 felt like pushing through cement, the last 1/3 flew by.
I was incredibly frustrated by the characters, most particularly Robin, throughout most of this installment. Her behavior was straight up irritating a lot of the time - her relationship with Strike was fraught and her relationship with Matthew was dysfunctional. The ending was the culmination of a series of misjudgments that made me want to slap her upside the head.
The mystery was a particularly grisly and disturbing, and the bits and pieces of insight we get from the mind of the killer were interesting. Rowling did a good job hiding the murderer in plain sight - her plotting is, as always, ingenious. There were points were I suspected pretty much everyone who showed up in the book, except for Strike and Robin.
I even had a few minutes where I thought "could it be Matthew?" before rejecting that possibility out of hand. But it's a testament to Rowling's ability to keep me off balance that I considered it.
I'm still thoroughly annoyed by Robin's behavior, but I can't wait until Lethal White shows up at the library with my name on it!