This is a meme of sorts that I saw at Duane's Abnormal Interests. Compared to Duane's oldest book from 1802, however, mine would be considered "new": Sanborn Tenney's 1872 A Manual of Zoölogy.
Tenney was a creationist. All but about 4 pages of the 535 pages of text in his book deal with descriptions of animals. In those remaining 4 pages, Tenney manages to refute evolution in his own way and concludes that "On the contrary, careful observers have been led to believe that animals as well as plants have been created by an Omniscient Being, in the places, and for the places, which they now occupy".
As Duane notes the "wonderful" illustrations in his old books, I value Tenney's book for the more than 500 engravings in it. This is one of them.
The oldest original copy of a scientific paper I own is George B. Simpson's 1901 "Anatomy and physiology of Polygyra albolabris and Limax maximus" published in the Bulletin of the New York State Museum. I have photocopies of much older papers, but they don't count.
Unlike Tenney's book, Simpson's paper is still quite useful for the unique and hard-to-find information it provides about the 2 species of terrestrial gastropods that were the subjects. The drawing below from Simpson's paper shows the "position of the pulmonary cavity, in relation to the volutions of the animal [Neohelix albolabris]". I have not seen a similar drawing of another species of snail in any other publication.
OK, here is a question. Why was zoology spelled as zoölogy until even the early 1960s? I don't know the answer.