This collection includes a letterbook consisting of 263 letters from other scientists to Mertens with copies of many of Mertens' replies handwritten onto the original letters. The letters are tipped into a bound volume. This includes letters from Carl Adolphe Agardh (1785–1859), J. B. G. M. Bory de Saint-Vincent (1778–1846), Augustin Pyramus de Candolle (1806–1893), Adelbert von Chamisso (1781–1838), Louis August Deschamps (1765–1842), Rene Louiche Defontaines (1750–1833), Ambrose M. F. J. Palisot de Beauvois (1752–1820), Alexander Postels (1801–1871), Dawson Turner (1775–1858) and many others.

Along with the letterbook, the collection includes: a personal notebook (cover title is Das Blaue Buch) belonging to Mertens with handwritten notes in German on many topics, including anecdotes, biographies, quotations, memoranda and caricature sketches; an ink-wash drawing, signed by Alexander Postels, of the corvette Senyavin on its circumnavigation of the world in 1829; a sketchbook by Mertens depicting a botanical field trip from Bremen to Bassum dated 25 July 1806 which includes three separate caricatures Mertens made on the trip; a pen-and-ink sketch of Mertens’ home at Bremen drawn by his son Carl Heinrich Mertens (1796–1830); and a satirical caricature titled "National Opinions on [Bonaparte]."