What kind of book ("folio volume") was in the middle of Dr. Heidegger's study in "Dr. Heidegger’s Experiment"?
In "Dr. Heidegger's Experiment," the rather eccentric Dr. Heidegger has invited four of his old (and miserable) friends to his study, a "curious place." It's full of many things, including volumes and volumes of books. The folio to which you refer is seen in the middle of the table and it draws everyone's attention even in the midst of so many other books. Hawthorne describes it this way:
The greatest curiosity of the study remains to be mentioned; it was a ponderous folio volume, bound in black leather, with massive silver clasps. There were no letters on the back, and nobody could tell the title of the book.
That's what it looks like. More importantly, though, this particular book has special properties which will be pivotal to the experiment to come.
But it was well known to be a book of magic; and once, when a chambermaid had lifted it, merely to brush away the dust, the skeleton had rattled in its closet
and other static elements in the room came to life. This is a book of magic, then, from which Dr. Heidegger does his magical work.