This book is a brilliant love letter to books and reading but with the unique position of not being from the perspective of a reader. There's something really powerful about reading about the importance and the impact of books while the MC himself doesn't really read. For the mystery, it's the same level as the previous two books.
The characters continue to be incredible, and the setting is still really fun to read and well researched. If you liked the previous two in the series, you'll continue to like this one. If you didn't, then I'm not sure this will do anything different to entice you. And if you're considering picking this up without the last two books, I would advise against it.