The Thomas Shadwell letter book consists of 90 correspondence items bound into a single volume. Shadwell wrote the letters to his friend, John Marsh, consul at Málaga, Spain, who collected them, bound them together, and wrote an introduction to them with a short note on Shadwell's background, dated March 25, 1791.
The letters span October 4, 1773-March 6, 1778, the period during which Shadwell worked as the private secretary to Baron Grantham, who was the British ambassador to Spain. Written from Madrid and from various localities nearby, including Arajuez, San Ildefonso, and San Lorenzo de El Escorial, they primarily relate to issues and happenings about British foreign policy, including the American Revolutionary War, the Spanish-Portuguese War of 1776-1777, the Russo-Turkish War of 1768-1774, Spanish-British relations, and Madrid's court gossip and social news.
Shadwell had particular interest in the Russo-Turkish War, and had lived in Turkey for some period of time prior to his arrival in Spain. He admired what he considered the "Purity of Morals & Simplicity of Manners amongst the Turks," and praised them for their lack of "wine and gaming." During his time in Turkey, he had met the eccentric scholar, Edward Wortley Montagu, whom he described as "an ingenious and a learned Man, but whose Moral Character I am afraid, is not without Stains of the deepest Dye" (April 5, 1774). In a number of letters, he commented on the progress of the war, and noted its conclusion in a letter of August 23, 1774.
Shadwell also commented frequently on Spain, its leaders, and its conflicts abroad. On April 11, 1775, he wrote about the birth of the future Queen consort of Portugal, Carlota, daughter of King Charles IV of Spain. He also made frequent references to General Alejandro O'Reilly, who had served as Governor of Louisiana in 1769, noting his activities, which included an expedition to attack Algiers, and relationships with other important Spanish figures. In several letters, he also tracked the events of the Spanish-Portuguese War of 1776-1777 as it progressed in South America, and on June 28, 1776, noted that "The Conduct of the Prime Minister at Lisbon has long been truely [sic] unaccountable." Shadwell also referred repeatedly to conflicts between the Spanish and the "Moors"; he predicted that the fort at Ceuta in Northern Africa would not be captured (October 31, 1774) and described Spanish distrust at Moorish efforts toward peace (April 11, 1775).
Shadwell closely followed the disintegrating relations between the British and the colonists in North America, first with a comment on the Quebec Act of 1774 and its protection of Catholicism in Canada, and later with contempt for the American patriots and their cause. On January 13, 1775, describing a proposal by the farmers of Virginia, he wrote, "The most laughable Resolution is that of the Virginians, not to plant Tobacco for our Use, for it will grow very well in England, and the planting of it there is prohibited solely in favor of them." On April 29, 1777, he expressed his hope that the Americans would "become thoroughly sensible of the nonexistence of their supposed Grievances" and predicted that the war would end within the year.
The volume closes with Shadwell's ongoing discussion of the failures at Saratoga, and he hopes that the British government will not be "intimidate[d]" by the capture of 4000 prisoners (January 13, 1778). He also mentioned that he planned to return to England to see his father (March 13, 1778).