More than the people listed below will have written about the subject (but not many) of heathen remains in Freemasonry, but I’ll list the people known to me who have written about the subject at some length (which differs from a few articles to massive books). I tried to give summaries of the theories in separate texts.
George Franklin Fort (1809-1872) is the first in our list. In 1884 he published Early History and Antiquities of Freemasonry, as connected with ancient Norse guilds, and the oriental and Medieval building fraternities. It is the first and one of the only brave attempts to show that some Masonic symbolism originates in Northern Europe. In his book he looks at old Germanic law and guild systems of the Middle Ages and how they merged with Italian building organisations to form some sort of proto-Masonry.
More about the book here. You can read some more quotes from the book here (search for Fort).
Egbert Jacob Smedes (1889-1975) was a Dutch Freemason, teacher, clerk and “modern humanist” who appears to be the first author who wrote texts exclusively about the particular subject. As far as I know, he didn’t write a book, but I do know of two (possibly three) lengthy articles. One was published in 1939 in the Masonic periodical Indisch Maçonniek Tijdschrift (‘Indian Masonic Periodical’) and is called Is Onze Loge een Directe Voortzetting van de Oud-Germaansche Gilde? (‘Is our lodge a direct continuation of the old-Germanic guild?’) The other article was published in a general spiritual magazine Mensch en Kosmos (‘Man and Cosmos’) in 1940 and is called De Vrijmetselaarsloge (‘The Freemasons Lodge’).
Farwerck (see below) also refers to another text in Mensch en Kosmos that he calls Germaanse Inwijdingen (‘Germanic initiations’), but I haven’t found that text yet.
Smedes knew Fort’s work, but was very critical about it. He found the approach too rational.
For Smedes’ theories see here.
Franz Eduard Farwerck (1889-1969) was the first author that I know who dedicated entire books to the subject. In 1953 he published Noord-Europese Mysteriën en Inwijdingen in de Oudheid (‘Northern-European Mysteries and Initiations in antiquity’) as B.J. van der Zuylen. In 1955 followed Noord Europa, een der bronnen van de Maçonnieke symboliek (‘Northern Europe, one of the sources of Masonic symbolism’), also as B.J. van der Zuylen. His final work was Noordeuropese Mysteriën en hun sporen tot heden (‘Northern Europe and their traces to the present’) in 1970 which was published posthumously under his own name.
Farwerck knew the work of Fort and Smedes, but his approach is far, far wider. His last book is 630 pages!
Much more about Farwerck can be found at Farwerck.nl.
For Farwerck’s theories, see here.
Henning Andreas Klövekorn (1975-). After a gap we come to contemporary authors. Klövekorn is a businessman, philanthropist, Freemason and Asatruar. He wrote one or two articles and his book 99 Degrees Of Freemasonry (2006) contains a chapter in which he describes how Freemasonry came from the German Steinmetzen. Actually a similar story as that “operative” Masons eventually grew into “speculative” Masons on the British isles, but then in Germany.
Klövekorn writes in English, contrary to most people mentioned on this page.
You can read the chapter from 99 Degrees here (external link)
Stephen Edred Flowers (1953-) is a prolific writer who mostly writes about runes, magic, etc., but in 2008 he published the small booklet Freemasonry and the Germanic Tradition.
He refers to Fort a lot (see above) and describes how he joined a lodge to find out if there are indeed links between the Germanic Tradition and Freemasonry, was soon disappointed and quit his membership.
Koenraad Logghe (1963-) is a Freemason and Asatruar and a productive writer. He wrote a lot that could be of interest for readers of this website, but most is not specific to the subject of this website. Also, much of what he writes is for a restricted audience (usually regular Freemasons).
In 2010 he self-published From Times Immemorial… de miskende wortels van de Vrijmetselarij (‘From Times Immemorial… the unknown origins of Freemasonry’). Also this work is only sold to regular Freemasons. The book is written in Dutch.
A bit more about the book here.
Arvid Ystad (1942-) is the last name on this list. Ystad was a Norwegian Freemason who was removed from the lodge that found his book too revealing. The book Frimurerne i Vikingtiden (‘Freemasonry in Viking times’) was published in 2016 in Norwegian. Work is being done on an English translation.
Ystad seems to not (or hardly) know any of the authors above, which is a bit of an absence in his book (it would have profited from the available information), but also makes that he has an original and thought-provoking angle to the subject. Since the book was published after this website started, you can find some information about it. Just search for Ystad.
Nice detail. Ystad has lectured at the study lodge Ars Macionica in Brussel, Belgium when Logghe was Worshipful Master there, 25 May 2019.
To get an idea of Ystad’s theories, see here and here (external link)