In his book, he asserts that any truly serious student of architecture, antiquity, and the arts must travel through France and Italy, and suggested that all "young lords" make what he referred to as the Grand Tour in order to understand the political, social, and economic realities of the world.
In 1637 he saw through the press at Paris his Latin translation, Epistola historica de mutuis officiis (dedicated to Charles I), of Smith's work, then visited Rome, probably on clergy business, and stayed there with Peter Fitton, a fellow priest and scholar.
During this time he translated the twelfth volume of Cardinal Caesar Baronius's Annales, which was published in Paris in 1639 as The Life or the Ecclesiasticall Historie of S. Thomas Archbishope of Canterbury.
But Lassels died in September 1668 in Montpellier in his capacity as travelling tutor, during what would have been his sixth voyage to Italy, accompanying Richard, Lord Lumley, and Ralph Sheldon.
His fellow priest Simon Wilson obtained the latest version of his Voyage manuscript, edited it (ultimately omitting those passages likely to offend Protestant taste), publishing it first Paris in 1670.
It is particularly notable for summarizing Vasari's Vite de' piu eccellenti pittori, scultori e architetti, as well as Carlo Ridolfi's Le Maraviglie dell'Arte and other Italian sources where relevant.
Aside from Lassels's published works, notes and two or three completed pietistic manuscripts ('Collections … out of Baronius' and the two-volume 'Apologie for the Roman Catholicks') in his fine, distinctive hand survive at Oscott College, near Birmingham.