Many details at http://revelatormagazine.com/from-the-vaults/brattleboro-days-yuggoth-ni...
Lovecraft was acquainted with Goodenough, and Lovecraft’s visits to Goodenough in Vermont in 1927 and 1928 are the basis of his wonderful novelette “The Whisperer in Darkness.” After the story was published in Weird Tales, Goodenough sent Lovecraft a congratulatory card, and also asked the author a couple of questions. Rather than responding with a card or letter of his own, Lovecraft wrote the answers in a tiny hand and then apparently gave the card to Vrest Orton — a bookman and eventual founder of The Vermont County Store — who returned the card to Goodenough personally during a trip to the Green Mountain State. Then Goodenough sent the card back to Lovecraft again, with follow-up questions written in a nearly microscopic hand. I suppose he knew the local postmaster, and was able to get the card back into the mail system without a problem. Amazingly, Lovecraft managed to fit the answers to the questions on the postcard in an even smaller hand. Sherwood told me that he’d guessed that Lovecraft used a magnifying glass and a sewing needle dipped in ink. Here’s an odd thing; Sherwood had found the postcard at an estate sale. It had been protected from the elements because it had been used as a bookmark in a 1935 number of The Revelator, and that number was a special issue dedicated to the “gothic tales” of Isak Dinesen.
It looks like an artifact from one of his stories: