The very first sentence of this blurb caught my eye and even if I weren't already a fan of the series, that alone would have drawn me in. Add in the setting and I was sold. I had the pleasure to visit Estes park years ago and it was gorgeous. After reading the book, I want to go back again and visit everything I missed the first time.
As for the story, Graham did it again. I have a few nitpicks but as a whole the story was gripping and entertaining. Graham did a fantastic job with the setting, making the reader feel as if they were truly there, capturing the beauty of the land and the distance from one place to the next. And the setting is integral to the mystery as it involves ranches, gold miners and US Marshalls in addition to our Scooby gang, the Krewe unit of the FBI. As always, the story is a blend of paranormal, mystery, and romance. The paranormal aspect and the mystery are predominant but she doesn't forget the romance. We met Diego previously, most recently in book 16, "The Forgotten".
The mystery was tightly written with several red herrings. I had several potential suspects whose billing as the top suspect changed as new information was revealed. In the end, the killer was someone I'd considered but discarded as a suspect. Well done. Ms.Graham!
Now the nitpicks; there are several. Granted, the Krewe is not your typical FBI unit and the agents use unusual methods but we still need believability. A suggestion is made by the agents that stretches credibility but not a single character blinked or thought it odd for a law enforcement officer to make. It's actually an idea I could see the Krewe doing on a regular basis; it was just odd that none of the non-Krewe characters thought was weird or even crazy for them to suggest.
Second, one of the local police officers did a 180 degree change in behavior and personality and we don't know why. A few of the characters note that he's behaving differently but it's left at that, no questioning of it. It was jarring. I kept waiting for a revelation and he was one of my candidates for the killer.
My last nitpick was the romance. I'm not a fan of second chance romances but sometimes they work. This one mostly did; the reasons they split up were believable as was their effort at reuniting and working through their problems. What felt off to me was how quick they were to fall in bed together. Yes, they'd been married before and weren't strangers but BOOM! they're in bed before working out their problems. I'd have preferred to see them talk and work through their problems before jumping in the sack.
Still, despite those niggles, "The Hidden" was enjoyable with strong writing, realistic and enjoyable characters (my heart broke for Nathan), an intriguing premise and set-up, and an entertaining mystery.