Writer and game designer Robin D. Laws brought you such roleplaying games as Ashen Stars, The Esoterrorists, The Dying Earth, Heroquest and Feng Shui. He is the author of seven novels, most recently The Worldwound Gambit from Paizo. For Robin's much-praised works of gaming history and analysis, see Hamlet's Hit Points, Robin's Laws of Game Mastering and 40 Years of Gen Con.
I've been working on a crossover Shadowrun / Earthdawn campaign for awhile, where half the session the players take the role of Earthdawn characters in the far past and the other half they play Shadowrun characters in the future in a story that spans both time frames. I'd been having trouble coming up with a compelling horror ("bug spirit" in Shadowrun parlance) to be a recurring thread between the two, but this sourcebook more than took care of that issue.
Other than the distinctive art, dice step system, and the way that magic was interwoven into every class, the major element that set Earthdawn apart from other fantasy settings was the horrors, which serve as much, much more than XP fodder in a dungeon. This look at the horrors really expands out what was in the core rulebook, with 15 major horrors explored in various ways, from direct retellings of people victimized to them to archives from scholars.
One horror brings you your very own "Needful Things" scenario in Throal, while another gigantic beast makes it exceedingly dangerous to either hide in a caer or try to escape by waterways. Probably my favorite is a horror that works against its brethren... for a price. There's a real devil's bargain to be made there, gaining intel on weaknesses and even magic items to specifically thwart a fellow horror at the low, low cost of your soul. Is it better to help spread sorrow and despair for this horror if it means killing a bunch of others?
The only real downside here for me was the compendium of minor horrors at the end, which are more like standard monsters from any given D&D / Pathfinder bestiary. Other than that, highly recommend anyone looking to run an Earthdawn campaign check this out.