James Boswell's uncle John was a member of a sect known as the Glasites.
Belle de Zuylen
Isabella Agneta Elisabeth van Tuyll van SerooskerkenBorn Oct 20, 1740
Died Dec 27, 1805 in Colombier, Neuchâtel
Daughter of Diederik Jacob van Tuyll and Jacoba Helena de Vicq. Married (1771) to Charles-Emmanuel de Charrière de Penthaz (1735-1808). Following her marriage she moved to Colombier near Neuchatel, after having lived in Holland for most of her life until that point. In 1760 she met David-Louis Constant d'Hermenches (1722-1785) who became a correspondent of both her and Boswell.
Zuylen gained some contemporary fame - especially on the continent - as a writer of belle lettres and poetry. Some of her most famous publications were Lettres Neuchâteloises (1784), Lettres écrites de Lausanne (1788) and the novels Trois Femmes (1795) and Sir Walter Finch et son fils William (1799). She also wrote operas and comedies, few of which were completed. Only few of her surviving works have, unfortunately, been translated into English, whereas most are readily available in both German and French.
Zuylen was one of Boswell's closest companions during his stay in Utrecht, and he considered (and rejected) marrying her several times during the course of their friendship. He first mentions her in a poem of October 31, 1763, which goes as follows
And yet just now a Utrecht lady's charms
Make my gay bosom beat with love's alarms.
Who could have thought to see young Cupid fly
Through Belgia's thick and suffocating sky?
But she from whom my heart has caught the flame
Has nothing Dutch about her but the name.
Let not an ear too delicate recoil
And start fastidious when I say "De Zoile";
So mere a trifle I can change with ease:
Your tender niceness will "Zelida" please?
On February 8, 1764, Boswell made "a pact of frankness [...] for the whole winther" with her - although he wrote, on February 9, that "[he] was angry for having thought of putting any confidence in her [...] be her friend, but trust her not", it seems that the pact actually lasted until they stopped corresponding in 1768.
Most of her works can be found via the AbeBooks.com used books search engine, among them the lettres mentioned above, and a 10 volume edition of her collected works, published in 1979-1981, entitled Oeuvres Completes. In 1906 Phillipe Godet wrote the 2 volume Madame de Charrière et ses amis, which was apparently a biography, including numerous illustrations, and in the 1920s came Geoffrey Scott's The Portrait of Zélide. Search for the titles, Zelide or Madame de Charriere.