I have read the book, many years ago. The show has some similarities to the book, which is a good basis. Frankly, if it was the same as the book there would be no value added to watch.
The show is engaging and very well done. It is a little darker, but the book was a little too light.
Yes it is a spy, supernatural agency show, with sufficient twists and turns to be interesting. It is not just fluff.
For those fans of Counterpart (another show unfortunately cancelled) you will definitely enjoy this, as I did.