I have some information on what I believe is the same Earl Pierce as your grandfather. I'm myself involved in the research of people who knew and/or corresponded with the author H. P. Lovecraft, and in this way I recently looked into (an) Earl Peirce with a colleague of mine. This Earl -- the full name would seem to be Earl Stanley Peirce, Jr -- was a one time (minor) pulp magazine author who (in his youth) knew Robert Bloch (the Psycho author) and also briefly corresponded with H. P. Lovecraft (apparently exchanging two letters in late 1936 - early 1937), after Lovecraft had commended favourably on Peirce's story "The Doom of the House of Duryea" (Weird Tales, October 1936) to Bloch. A short bibliography of his published fiction can be found here: http://www.isfdb.org/cgi-bin/ea.cgi?17035
(My research colleague tells me that Peirce moved from writing weird fiction to detective tales.)
Here's a summary of what else we could find out about Peirce (the identification of him with your grandfather would seem to be pretty certain, based on what you have said here):
United States Census, 1930: Syracuse, Onondaga, New York (enumeration district 0108, family # 355, sheet 19A, line 1): 300 Kensington Road, Syracuse, NY
United States Census, 1940: District of Columbia (enumeration district 1-299, family # 207, sheet 61B, line 78): 3738 Huntington Street, N.W., Washington DC
Both list the family as father Earl Peirce, mother Dorothy B, and sons Beach, Earl J[r], Dudley and Peter (all unmarried in 1930/40 and born in Wyoming). The 1930 census also gives a servant from Scotland, Mary Campbell. Earl Sr. was born in Maine (as his parents) and Dorothy in Connecticut (as her parants). In the '30 census the occupation & industry of the father is given as "broker / investment" and in '40 as "assistant chief of state & [_] of forestry / US forest service"; Earl Jr's as "magazine writer / writing" in 1940.
Earl Sr and Dorothy were married in 1913, as shown by the South Dakota State Census of 1915.
The marriage of Earl Jr. and Gloria Hallet Grimm can be found in "District of Columbia Marriages, 1811-1950", having taken place on 26 Dec 1941.
In SSDI I found entries for Earl Peirce Sr and Jr with the sos.sec. numbers 579-60-8149 and 577-26-1313, respectively:
Sr: b. 26 September 1886, d. November 1978
Jr: b. 28 February 1917, d. June 1983
Earl Peirce Sr worked mainly for the US Forest Service, and there are some publications online, one of which contains this biographical info on page 11, including "September 26. 1886 / Born at Frankfort, Maine" and : Salvage programs following the 1938 hurricane (http://archive.org/stream/salvageprogram6800peirrich
Another source is "Biographical record of the graduates and former students of the Yale Forest School; with introductory papers on Yale in the forestry movement and the history of the Yale Forest School (1913)" (http://archive.org/stream/biographicalreco00newh
), where p. 299 contains a short biography of EP Sr. (The year of birth is off by one in this book.)
As for EP Jr, we couldn't find that much besides the genealogical details, vital statistics and pulp stories, but Robert Bloch (to whom H. P. Lovecraft had remarked in a letter in June 1935 that "Young Peirce seems to be a very interesting character, & I surely wouldn't mind hearing from him some day.") is on record with some comments:
"... upon becoming a professional writer I began to receive communications from readers with aspirations of their own. One such was Milwaukee resident Earl Peirce, Jr.; I encouraged his successful submission of stories to Weird Tales." (Once Aroud the Bloch: An Unauthorized Autobiography, p. 204)
"I knew Earl Peirce Jr. in Milwaukee as a fan in 1935-37. He was a bright personable young man, about my age, whose father was in the U.S. Forestry Department. He contacted me, expressing an interest in writing, and I encouraged it – introducing him to my circle of friend and (via mail) to various writers I knew. He wrote and sold several stories – "Doom of the House of Duryea," a vampire yarn, was his best – then moved to Washington with his family. In late '41 I visited him there with my friend Harold Gauer: he had married and was (I seem to recall) working for the Navy Department. That was the last I saw or heard of him for at least twenty-five years. Then he showed up here, with a different wife, and spent a day with me. He had changed so much that I'd never have recognized him, and there wasn't a trace of the rather intense and imaginative fantasy devotee who had once dreamed of starting an organization to rule the world – the "Si-Fan," modeled on Sax Rohmer's secret society in the Fu Manchu series." (interview with Graeme Flanagan, http://mgpfeff.home.sprynet.com/flanagan_interview1.html
There's also a letter from Peirce himself to the letters column of the Weird Tales magazine after H. P. Lovecraft's death in March 1937:
"The news of Lovecraft's passing, although not the shock of surprise, is nevertheless the shock of an irreparable loss, not alone to WT, but to his admirers and acquaintances the world over. I shall always regret that I never had the good fortune of meeting him personally, but I am truly grateful for the impulse which prompted me to write to him a few months ago, and that I have two letters in his own hand. What most impressed me were his sincerity and genuineness, which qualities were not alone in making him unique among modern writers. You have my sympathy, for this must be a hard time, but I imagine it is a feeling of pride for you to know that so many of his stories originally appeared in WEIRD TALES. Unlike many other men of genius, Lovecraft was fortunate enough to be living at a time when his work was recognized as outstanding. With the passing of time this recognition will become more universal and his work will take its proper place in the world's great literature."
I can send you a text file with somewhat fuller details and reference links and copy-paste of relevant details, if it would be of use. I would also be interested in any further information that you might yourself have.