The Mythos Dossiers is an interesting supplement / sourcebook. The presentation and content of the sourcebook to emulate actual files that would be found in the intelligence dossiers. Like the novel, these files are codenamed rather interestingly: BLUE HADES, ANNING BLACK, EQUUS STELLAR, PLUTO KOBOLD, etc. and are more or less consistently used throughout the sourcebook across all 'files', reinforcing the feel of working for a bureaucratic agency dealing with the supernatural.
The formats of the files found in the dossiers are many and varied: there are letters, handwritten and typewritten; there are transcripts of recorded interviews; there are photos that are vague yet suggestive; there are diagrams; there are situation reports and assessments; and so on.
Some of them are quite humorous, such as the BLUE HADES interview; others are terrifying in their implications. Even better, the more files that you read in each dossier, the more you learn about each topic -- players and GMs may end up drawing connections between things that they wouldn't have seen taking each file individually.
GMs can use this sourcebook as inspiration for scenarios and campaigns. They can also hand out entire dossiers grouped by codename to get PCs ready for a scenario tackling that dossier. They can also hand out files piecemeal, allowing the PCs to learn more and more as an adventure or scenario progresses.
I love this sourcebook -- it feels true to both inspirations: the classic Call of Cthulhu handouts and The Laundry novels.
(from my online review at armchairgamer.blogspot.com)