As I went deeper into my research, I found it difficult to find secondary literature on the history of botany. The most recent comprehensive textbook I could find was from the 1980s. Histories of biology, which tended to focus more on zoological subjects, seemed to be much more plentiful. A result, I began keeping track of almost anything I could find. Now I see that there is a lot that has been written, though I still have not found any recent comprehensive works. Below are the results of my search, which I plan on keeping up-to-date. If you know of any books that I am leaving out, please contact me.

General Histories

• Sachs, Julius. Geschichte der Botanik vom 16. Jahrhundert bis 1860. München: R. Oldenbourg, 1875.
• Sachs, Julius. History of Botany (1530-1860). Translated by Henry E. F. Garnsey and Isaac Bayley Balfour. Oxford: Clarendon Press, 1890 and 1906. 
• Green, J. Reynolds. A History of Botany, 1860-1900: Being a Continuation of Sachs “History of Botany, 1530-1860.” Oxford: Clarendon Press, 1909.
• Greene, Edward Lee. Landmarks of Botanical History: A Study of Certain Epochs in the Development of the Science of Botany. Washington: Smithsonian Institution, 1909.
• Lloyd Library. Bibliography of Botany. Cincinnati :  Lloyd Library, 1918.
• Harvey-Gibson, R. J. Outlines of the History of Botany. London, Black, 1919.
• Reed, Howard S. A Short History of the Plant Sciences. Waltham: Chronica Botanica, 1942.
• Morton, A. G. History of Botanical Science. London: Academic Press, 1981.

British

• Oliver, Francis Wall. Makers of British Botany. Cambridge: University press, 1913.
• Green, J. Reynolds. A History of Botany in the United Kingdom from the Earliest Times to the End of the 19th Century. London: J. M. Dent and Sons, 1914. 
• Bower, F. O. Sixty Years of Botany in Britain (1875-1935). London: Macmillan and Co., 1938.
• Gilmour, J. S. L. British Botanists. London: William Collins, 1944.
• Allan, Mea. Darwin and His Flowers: The Key to Natural Selection. New York: Taplinger Publishing, 1977.
• Desmond, Ray. Dictionary of British and Irish Botanists and Horticulturists Including Plant Collectors and Botanical Artists. London: Taylor and Francis, 1977.
• Walters, S. M., and J. S. Henslow. The Shaping of Cambridge Botany. Cambridge: Cambridge University Press, 1981.
• Gascoigne, John. Joseph Banks and the English Enlightenment. Cambridge University Press, 1994.
• Shteir, Ann B. Cultivating Women, Cultivating Science: Flora’s Daughters and Botany in England 1760-1860. Johns Hopkins University Press, 1996.
• Stearn, William Thomas. John Lindley, 1799-1865: Gardener-Botanist and Pioneer Orchidologist: Bicentenary Celebration Volume. Woodbridge: Antique Collectors’ Club, 1999.
• Ayres, P. G. The Aliveness of Plants: The Darwins at the Dawn of Plant Science. London: Pickering & Chatto, 2008.
• Endersby, Jim. Imperial Nature: Joseph Hooker and the Practices of Victorian Science. Chicago: University of Chicago Press, 2008.
• Chang, Elizabeth Hope. Novel Cultivations: Plants in British Literature of the Global Nineteenth Century. Charlottesville: University of Virginia Press, 2019.

North American

• Rodgers, Andre Denny. American Botany, 1873-1892: Decades of Transition. Princeton: Princeton University Press, 1944.
• Humphrey, H. B. Makers of North American Botany. New York: Ronald, 1961.
• Ewan, Joseph Andorfer, ed. A Short History of Botany in the United States. New York: Hafner, 1969.
• Wetmore, Ralph H. Irving Widmer Bailey. Washington, D.C.: National Academy of Sciences, 1974.
• Volberg, Rachel Ann. “Constraints and Commitments in the Development of American Botany, 1880-1920.” University of California, 1983.
• Dupree, A. Hunter. Asa Gray: American Botanist, Friend of Darwin. Baltimore: Johns Hopkins University Press, 1988.
• Smocovitis, Vassiliki Betty. “Botany and the Evolutionary Synthesis: The Life and Work of G. Ledyard Stebbins Jr.” Cornell University, 1988.
• Johnson, Victoria. American Eden: David Hosack, Botany, and Medicine in the Garden of the Early Republic. New York: Liverwright Publishing Corporation, 2018.
• Lannoo, MIchael J. This Land Is Your Land: The Story of Field Biology in America. Chicago: University of Chicago Press, 2018.
• Mauz, Kathryn. C. G. Pringle: Botanist, Traveler, and the “Flora of the Pacific Slope,” (1881–1884). New York: New York Botanical Garden Press, 2018.