The Raven King is the fourth and final book in The Raven Boys cycle and they should be read in order. Gansey and his friends are still looking for Glendower, but now things are starting to spin out of control, to literally unravel. Stiefvater excels at creating a feeling of entropy and dread, but I sometimes felt like the plot was doing the same. Magic that has been proven real, suddenly stops working, characters seem to float in and out of the storyline and the climax was, well, less than climactic.
But I'm still going to recommend this read. Stiefvater is a master of atmosphere-- creating a sense of sinister dread-- but also of character. Watching Blue and her boys try to navigate their conflicting needs and aspirations is like listening in on the conversation of real teenagers. Fear and hope for the future vie with a need to reconcile childhood and adulthood. If you've loved the group so far, you won't be disappointed with how they progress, but you may be less excited in the resolution of the central mystery.