Lurker at the Threshold (Vintage Paperback) by H.P. Lovecraft, August Derleth

Ambrose Dewart is the heir to the fortunes of his illustrious forebears. Having sampled the wide world and its pleasures, it's now time to retire to the ancestral estate deep in the rolling hills of Massachusetts. At once he sets to work refurbishing the dark rooms, clearing the cobwebs and brightening the foreboding atmosphere. That's when he stumbles upon an old document handed down from a great-grandfather of sinister reputation. On it reads the ominous injunction: Never invite him that lurks at the threshold!

But this caution does not suppress Dewart's inquisitive spirit... not even when fantastic, ethereal powers are near at hand, striving to resume their earthly domination....