Coturnix the Chronobiologist offers an oldest book meme. I guess my library is in as much disarray as his - especially as it's split between home and office - so here's what I have to hand and own that pre-dates 1900:
- Geology and Revelation: of The Ancient History of the Earth Considered in Light of Geological Facts and Revealed Religion. Gerald Molloy, 1873 (2nd ed).
- Memoir of Robert Chambers with Autobiographic Reminiscences of William Chambers. William Chambers, 1872.
- Typical Forms and Special Ends in Creation. James M'Cosh and George Dickie, 1857.
Not very much, I know. I like old books, I just can't afford them!
After 1900, I've a first edition of D'Arcy Wentworth Thompson's On Growth and Form if that counts for anything.