The full title is Archaeological Interpretations of Myth Patterns in the Iconography of the Codex Beltran-Escavy (Miskatonic University Press), and at ten bucks it is one of the best Call of Cthulhu RPG props I’ve ever seen that isn’t actually explicitly a Call of Cthulhu RPG prop. The conceit is that the pamphlet is a scholarly monograph describing and analyzing a pre-Columban Codex depicting a hitherto unknown deity; while the scholars themselves are apparently blissfully ignorant of what is actually being portrayed in the pictographs, any halfway competent Mythos scholar (and/or person who plays Cthulhu Mythos games) will instantly catch on to the Codex’s true meaning. You can easily craft several adventures around this monograph, with it as the central prop.
The artwork is also very nice. There are color drawings and a fold-out centerpiece. The HPLHS really worked on this one.