The first fifteen volumes of these "Notes of Travel" are Lea's account of his trips to Europe during 1832 and 1852-1853. He met with naturalists, scientists, and did much sightseeing in England, France, Germany, Italy, and Austria. He records in detail visits and discussions with scientists, and makes sketches of many of the places he saw, particularly the Rhine River. The sixteenth volume is a memorandum book, 1853, containing accounts and expenses while in Europe.

These letters concern various scientific subjects, such as fishes, insects, snakes, mice, fossils, and mosses. Other subjects include learned institutions, notably the Smithsonian Institution and the Academy of Natural Sciences of Philadelphia, and men of science. Correspondents include S. S. Haldeman, Walter J. Hoffman, Spencer F. Baird, Edward D. Cope, and Leo Lesquereux.

The primary correspondence (ca. 65 letters) is with William Cooper (1798-1864) on Bonaparte's publications, especially "American Ornithology" and "Observations on the Nomenclature of Wilson's Ornithology," but also included are many references to American and European men of science and learned societies. There are also letters to Isidore Geoffroy Saint-Hilaire, George Robert Gray, Titian Ramsay Peale, and Wilhelm P. S. Rüppell.