Jena's extraordinary intellectual history means there is no shortage of reading material to bring to the city. The Romantic and idealist philosophy period in particular has produced a rich body of scholarship.
The Romantic Generation by Charles Rosen — not specifically about Jena but essential for understanding the intellectual atmosphere of the 1790s when Schiller, the Schlegel brothers, Goethe, and Fichte were all in the Jena–Weimar orbit simultaneously. Rosen shows how literary, philosophical, and musical ideas cross-pollinated in this period.
Goethe: Life as a Work of Art by Rüdiger Safranski — the fullest account in English of Goethe's life, which is inseparable from Weimar and Jena. His relationship with Schiller, his botanical work in the Jena garden, and the Phenomenology of Spirit period all feature.
The Metamorphosis of Plants by Johann Wolfgang von Goethe — Goethe's actual scientific essay, developed from observations made in the Jena botanical garden, illustrated with his own drawings. Short, strange, and unlike anything else.
Art Forms in Nature (Kunstformen der Natur) by Ernst Haeckel — the illustrated plates that made Haeckel famous and influenced a generation of Art Nouveau designers. Still in print and still startling.