A new 120-page book claims to catalogue all the monsters of Robert E. Howard. Conan: Horrors of the Hyborean Age appears to be one of those PDF books for gamers that that give them the monster ‘stats’, but which are also rather useful for the reference shelves of writers.
Not sure about the cover, though. I recently re-read the Howard Conan stories in audiobook and I don’t quite remember Wonder Woman fighting a T. Rex, as per this book’s cover. Nor the distinctly LOTR orc who flanks Conan.
As a gamebook it needs to be interflipped with the Robert E. Howard’s Conan: Adventures in an Age Undreamed core rule-book. There appear to be other catalogue-like guide books to Conan’s world in the same series, one on Ancient Ruins & Cursed Cities and a guide to Nameless Cults, Cosmology and Gods. Apparently they all have inspiring art inside, and I’d guess also some maps.