Dubious Shards, written by Ken Hite, is a collection of essays on different aspects of the Cthulhu Mythos. Hite covered a very wide spectrum of topics relating to H.P. Lovecraft's literary work from Dagon to vampirism with an unreleased Delta Green scenario included for good measure.
The essays are well-thoughout and researched. The two that stood out for me the most are The "How" of Hastur and The Man Who Shot Joseph Curwen. Both deal with the Call of Cthulhu Roleplaying Game. Hastur, explains how Lovecraft composed mythos stories by fusing his interests with real life events. The Man Who Shot Joseph Curwen compares Call of Cthulhu investigators to the gunslingers of the western genre with the intent of giving an answer to CoC's lifelong dilemma: "why do investigators investigate." These two scenarios should be read by every Of Cthulhu keepers or players that wish to take their games beyond the introductory one-shots like Dead Man Stomps. They are invaluable for anybody interested in writing scenarios.
The other essays trace creatures and events present in Mythos stories to their forkloric and mythological origins. They offer valuable literary studies for people interested in writing mythos stories. (And Keepers that REALLY take their job seriously.)
I haven't read the Delta Green scenario, The Winslow Project, because there's no Delta Green GMing in my horizon for the time being.
The only flaw I can find with the book (after purposely looking for one) is the short lenght of it's essays.
Dubious Shards transcends the RPG genre into full blown literary study comperable to anything writen by S.T. Joshi.