David Dunham Page
Delta Green provides information on the Delta Green organization (which has changed dramatically since it was first formed), and four others, which investigators may have to deal with (either as friends or foes). And this doesn't count the Greys. There's also information on federal agencies, new skills, a compendium of firearms, two scenarios and a short campaign, and a character sheet. Along the way, important secret places such as Area 51 are detailed.
Delta Green provides a framework for investigators to cooperate in a long-running campaign, and is a perfect excuse not to worry about the details of having resources for an investigation. But the history of the organization also provides some important constraints (as well as some recurring opposition). Many of the important members are detailed, but there's plenty of space for the player characters.
The opposition (and related conspiracies) all make sense, and fits nicely into the Cthulhu mythos. The scenarios dealing with them are reasonably elaborate, and range from relatively easy to difficult bloodbath. Handouts are provided, both to add flavor, and as a convenience.
The book is remarkably free of typos, and there's an index.
The book falls prey to the current fad of large, decorative but useless page ornamentation (it could have been used to make chapters easy to find). And I suspect having an ongoing campaign is a risk to Delta Green's cell-style organization.
Delta Green is certainly a weighty supplement, but there's no fluff. If you're at all interested in running a modern-era Call of Cthulhu type game, you should have it. You could easily adapt most of the organizations to a non-Chthulhoid conspiracy if you're so inclined, or have players involved in one of the groups other than Delta Green. But even if you don't use Delta Green itself, the numerous federal investigator NPCs and the weapon list will be handy.