I read a novel about 15 years ago about an ancient book, which when read, progressively opened a portal to a demon like world. A researcher was given the book to translate and the more he read it, the further he ventures in his mind into this sort of dream world allowing demons through with "wings sounding like flapping leather" . Does anybody know the author/title please?
Could it be The Haunter of the Dark
hplovecraft.com/writings/texts/fiction/hd.aspx
The protagonist is a reporter though and though several ancient books are mentioned it's not really an ancient book that opens a portal. Anyway, here is a quote from the story that might help refresh your memory:
It was in June that Blake’s diary told of his victory over the cryptogram. The text was, he found, in the dark Aklo language used by certain cults of evil antiquity, and known to him in a halting way through previous researches. The diary is strangely reticent about what Blake deciphered, but he was patently awed and disconcerted by his results. There are references to a Haunter of the Dark awaked by gazing into the Shining Trapezohedron, and insane conjectures about the black gulfs of chaos from which it was called. The being is spoken of as holding all knowledge, and demanding monstrous sacrifices. Some of Blake’s entries shew fear lest the thing, which he seemed to regard as summoned, stalk abroad; though he adds that the street-lights form a bulwark which cannot be crossed.
Of the Shining Trapezohedron he speaks often, calling it a window on all time and space, and tracing its history from the days it was fashioned on dark Yuggoth, before ever the Old Ones brought it to earth. It was treasured and placed in its curious box by the crinoid things of Antarctica, salvaged from their ruins by the serpent-men of Valusia, and peered at aeons later in Lemuria by the first human beings. It crossed strange lands and stranger seas, and sank with Atlantis before a Minoan fisher meshed it in his net and sold it to swarthy merchants from nighted Khem. The Pharaoh Nephren-Ka built around it a temple with a windowless crypt, and did that which caused his name to be stricken from all monuments and records. Then it slept in the ruins of that evil fane which the priests and the new Pharaoh destroyed, till the delver’s spade once more brought it forth to curse mankind.