The Pathfinder Bestiary, and the long line of monstrous encyclopedia that came before it, do a great job of attacking the problem of providing game statistics and mechanical information for a very wide variety of creatures. There are a lot of strange creatures from fantasy novels and movies, from ancient mythology, and, as time went on, from the twisted minds of game designers and even GMs, for that matter. Anyone today who tried to go about creating an extensive bestiary of all the different cool and interesting fantasy monsters could fill three or four volumes with three hundred monsters each, and still have a long way left to go, with fans crying out for various "forgotten" monsters to get some attention. That kind of breadth doesn't leave a whole lot of room for depth, and that's where Exotic Encounters comes in.
There are all kinds of reasons why you might not want to use a monster stat-block straight from the core rules. For one thing, if your players have a habit of browsing through such books, or are long-time veterans who know the basic ins and outs of most fantasy staples, you may need an unusual stat-block just to throw them a curve ball and teach them that they can't count on their out of character knowledge to take all the mystery out of the game. Alternatively, many GMs can recall a time when they wanted to make use of a certain monster in their game, only to find that it was a few CRs too high or low for what they had in mind.
Exotic Encounters takes a single, iconic monster, and creates three new variants on that theme. These variants aren't simple tweaks, and are more than simply advancing or removing Hit Dice, though that occurs as well. Each of these three variations on the monster's theme has a specific, flavorful goal in mind, and a role to play, and their statistics entries are gently massaged in order to make them fit those roles. Further, each of these variants comes with brand new specially-crafted special abilities, which are unique to Exotic Encounters and not found anywhere else.
This particular installment of Exotic Encounters focuses on the wraith. While the numerous spectral undead can be somewhat difficult to keep track of (the category includes, but is not limited to, allips, banshees, ghosts, poltergeists, shadows, spectres, and wraiths), each of the wraiths in this tome is formed when a creature dies under certain circumstances, and is animated by the rage or despair surrounding the circumstances of its death. The first wraith in the book, the plague wraith (CR 7), is created by plague victims who are shut away and left to die by the healthy. They carry a number of deadly diseases, including the dreaded wraith's wasting, which slowly transforms the victim into a plague wraith himself, and they can possess the bodies of those suffering from diseases. The second wraith is the lifedrinker wraith (CR 9), whose touch instills a deadly curse that blocks healing, and which can drain the life force from victims in a 30-ft.-cone. Finally, there is the doom wraith (CR 14), which spawns large number of standard wraiths from the creatures it kills, and can leave foes in an apathetic stupor, though it can be fought by the power of concentrated hope and goodwill.