Okay, when J. K. Rowling writes, she uses a lot of words. This has been happening since book four of the Harry Potter series, and it carried over into this series.
Except, in this book, I didn't feel like it had too many words. So either I've become used to too-many-words, or Rowling's editor managed to convince her to trim the excess with this book.
I enjoyed this book. It isn't a short book, taking most people 11 hours to read, it took me a couple hours less, but that's still a lot of words.
The book starts off pretty much where the last one ended, Robin's wedding, and does a number of flashbacks and fast-forwards to tell the story of that day, week, month. The first part had a number of miscommunications and interfering people and George Costanza awkwardness that I almost stopped reading it. I kept going, and enjoyed the book.
There are a couple mysteries in the book, with the bulk following an unhinged Billy as he deals with memories of childhood violence that may or may not have happened, and a separate investigation of the blackmailing of a government official. I will admit to having spent most of the book trying to figure out what would have been legal six years before, and is no longer legal, and why that would be blackmailable. I suspect this was part of the delight of the mystery.
The Cormoran / Ellacott partnership works well professionally, but both of them long for the other in the book, which leads to a stressed professional relationship. I guess this makes for good tension in a mystery book? Maybe?
Anyway, if you're reading the series, keep going. If you haven't started, start at the first one and make it through the second one. These last two have been fun. Not Harry Potter books 1-3 fun, but adult action-mystery fun.