This came together well as an ending to the series. There were a couple of surprises - Charlie's sister, the identity of The Dire King (I pretty much figured this out), and a few loose ends that were never wrapped up - Douglas stays a duck? Overall, though, I thoroughly enjoyed the entire series.
If pressed, I think that the second book was the weakest. The historical urban fantasy element was fun, and I would read more books with sort of an aesthetic - it reminded me a little bit of the Fantastic Beasts franchise from a visual sense - merging early American elements with paranormal investigation and a strongly feminist sensibility.
There is some room at the end for additional books in Jackaby's world of New Fiddlesham, but whether William Ritter stays in the familiar place he's created or moves to write about someplace else, I'll give whatever comes next a try!