A fantastically detailed and rich sourcebook for the Pendragon RPG: possibly the only sourcebook you'll ever need.
The campaign is served up year by year, with detailed rundowns of what happened during that year, what people were gossiping about, changes to technology, adventure hooks, and so forth. Even if you are not a player of Pendragon - gasp! - the GPC is a pretty good reference text to what happened in the Arthur myths. Full stats for the major NPCs are also downloadable from the publishers website.
Th production values are quite good too, with a consistent look and feel and - most importantly - tone throughout the text. The PDF is of excellent quality, with a full set of bookmarks.