Guillemard's 80 or so letters to his family from America include correspondence primarily with this uncles, James and Isaac Guillemard, merchants in London. There are also some letters addressed to his sister, Jeanne Marie Griffin, and a few to Guillemard's nieces Frances, Jane, and Mary Griffin. There are also a couple of letters to and from Guillemard's brother-in-law, Davies Giddy. The letters are written principally from Philadelphia (Pa.), including "The Solitude," John Penn (grandson of William Penn) and his estate located on the outskirts of the city; the French émigré colony of Asylum, (Pa.); New York; Boston; Quebec; and Montreal.

Guillemard's detailed and insightful letters document a number of topics, in particular political happenings in the country as well as America's foreign relations with France and England following the Revolutionary War. Guillemard himself becomes involved in the latter when he is asked in May 1797 to serve as the fifth Commissioner to the Jay Treaty to deal with British property claims in America following the war. (While serving, Guillemard makes sure that his uncles do not forward any claims to him personally so as to avoid any hint of bias.) In his last letter dated May 6, 1800, Guillemard discusses America's current political state: "The administration here is in great confusion. The President has removed the Secretary of State, and the Secretary of War has resigned, and the Attorney General executes these offices. It is pretty clear that Mr. Adams does not even expect to be re-elected President, and it is probable Mr. Jefferson will. The measures of the... Men will be not less hostile to England than those of the present party, but they will perhaps have an air of being so, because the hatred to Great Britain which is now hypocritically concealed will then be openly avowed." His reflections on the future of the country are prescient: "This country is much divided in its real and supposed interests. – some of which are founded on geographical situation, some on commercial calculation, some on imaginary or real wrongs. Of these the former are the most likely to break up the Union, but it will not be for many years (May 2, 1797)."

Besides documenting American politics, Guillemard's letters also include observations and information about American society, bankruptcies and financial failures, the activities of land speculators, local insurrections against "oppressors" and tax-gatherers, and outbreaks of yellow fever, which during his stay in Philadelphia, resulted in his taking up residence at "The Solitude". His correspondence also relates to personal matters such as family and financial affairs, including Guillemard's interest in purchasing an estate in England.

Guillemard's letters also include discussions and observations of his travelling companion, the French social reformer François Alexandre Frédéric, duc de La Rochefoucauld-Liancourt. On March 25, 1795, Guillemard writes that he is going to Canada, "with a gentleman who has spent much time in England, whom Fortune once smiled upon...Now that fortune frowns upon him he is still manly and resigned, melancholy at times, but at others enjoying with unfeigned pleasure the good which are left to him." However, de Liancourt was refused entry: "The gate of Paradise was shut upon him", Guillemard supposes in order to "keep him from intercourse with the native Canadians." Guillemard also traveled with de la Rochefoucauld to the small French émigré colony of Asylum in northern Pennsylvania ("On the E. Branch of the Susquehanna, about 180 miles N.W. of Philadelphia"), "an establishment originally imagined by some Frenchmen – with the desire of collecting together the ruined French who have been driven, or have escaped, from France or the French Islands…(May 28, 1795)" (Rochefoucauld documented his American travels in Journal de voyage en Amerique et d'un sejour a Philadelphie, 1 octobre 1794-18 avril 1795.)

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These 14 letters written from Paris, Nancy, Strasbourg, Besançon, Geneva, Zurich and elsewhere and addressed to Guillemard's sister Jeanne Marie Griffin and his father Jean Guillemard, document two of Guillemard's journeys-- the first in 1787-1788 and the second in 1789-- to France and Switzerland prior to and at the outset of the French Revolution. Also included are three typed copies of Guillemard's letters written circa August and September 1789 to Newton Ogle, aide-de-camp to Earl Grey, with whom he witnessed the aftermath of the capture of the Bastille. There is also an account Guillemard wrote in 1823 detailing his experiences in France with Ogle, which includes documentation of an attack that occurred while they were staying in Roeun, and their witnessing in front of the Palais Royale the procession transporting the heads of Jacques de Flesselles and Bernard René de Launay.

Guillemard's letters during his first trip to France mostly describe tourist activities, such as visits to monuments, buildings, and performances he attended; however, they also address signs of unrest, which are made clear when an English officer orders him to leave the country. After leaving Paris, Guillemard traveled to Switzerland. In these letters, he offers accounts of fellow travelers and persons with whom he visits, including Prince Edward, Duke of Kent and Strathearn, and Swiss poet and physiognomist Johann Kaspar Lavater; and also discusses the trial of Warren Hastings.

Guillemard's letters documenting his second trip to France with Newton Ogle offer detailed accounts of their experiences. Writing to his sister from Paris on July 17, 1789, a few days after the storming of the Bastille, Guillemard notes, "Yesterday was a day of great distress; on Wednesday Evening deputies from the National Assembly arrived in order to dissipate the fears of the Parisians and announce the King's intention of visiting his Capital...He came today in great form, attended by the National Assembly on foot, and guarded by the Militia of Versailles...There were above 400,000 Citizens and others under arms...When he entered a dead silence prevailed...The common Guards of Paris are disbanded, and a Militia composed of Citizens perform the office of guarding the Place, and preventing disturbances. The Bastille will I hope never been seen again. It is now level with the ground." In a postscript written from Geneva on July 25, Guillemard adds, "I wrote this letter at Paris, and meant to have sent it from Versailles, but I have not been able. The whole kingdom is in a tumult not to be easily described... We have arrived safe at Geneva after much care and trouble."

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Consists of about 50 letters to Guillemard's nieces, Frances Griffin (d. 1868), Jane Griffin Franklin (1791–1875), and Mary Griffin (1793–1854), from London, Tredrea (the Cornish home of Guillemard's brother-in-law Davies Giddy), Clavering, Paris, Florence, Rome, Naples and elsewhere.

Childless himself, Guillemard treated his nieces as though they were his own children. He was very interested in their education, and his letters include discussion of ideas and developments in the intellectual milieu in which he moves; book recommendations; and reports on meetings with the savants of Europe, Constantin François de Chassebœuf, comte de Volney in particular.

His letters document his time on the Continent, primarily France and Italy, between 1814 and 1818. While in the former, he stays with his friend, François Alexandre Frédéric de la Rochefoucauld-Liancourt, renews his acquaintance with some of the country's aristocratic families, and visit scientists and a number of the English residents in Paris. Letters of his trip to Italy include descriptions of the Neapolitan War.

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