The ill-fated mountaineer George Mallory's only book was "Boswell the Biographer"?
Our knowledge of des Essar stems almost entirely from Boswell's French Theme of March 31, 1764, in which he tells us that "he was once a Capuchin [who] ran away twice from his monastery, and the second time he escaped to Brussels, where he lived for some time. He taught mathematics to people of the first rank. [...] Essar is assuredly of a distinguished family, though of a rather distant branch of it. [...] He established himself at Amsterdam [where he had] pupils of the right sort. He belonged to a literary society composed of men of wit and amiable women.... Finally monsieru des Essar married a young Hollandised Englishwoman. He came to Utrecht, where he has remained for some time. He still teaches mathematics, and he conducts the Gazette Francaise."
Life with Boswell:
Boswell probably knew des Essar though the Literary Society of Utrecht. Des Essar is mentioned four times throughout Boswell in Holland 1763-64.