Micrographia by Robert Hooke. A CD ROM version of the original monumental volume, complete with Hooke's 117 meticulous illustrations. Hooke, one of the earliest microscopists, published this volume in 1665. Stunning!
Experiments in Electricity By Benjamin Franklin. Another brilliant facsimile edition on CD ROM. This is arguably the most important book on electricity of the 18th century, and one of the founding studies on the subject. Goes way beyond the "lightning rod" experiments.
The Meaning of Relativity by Albert Einstein is a collection of lectures Einstein delivered in 1921 at Princeton to describe his theory of relativity. This is the clearest description of Einstein's theorys as penned by Einstein himself.
On the Revolutions of Heavenly Spheres (Great Minds Series) by Nicolaus Copernicus. The masterwork from the guy who put the sun at the center of the solar system; up until this time, popular opinion was that the earth was the center of the universe.
The Octavio CD rom facsimile edition of Sidereus Nuncius is also highly recommended!
Principles of Geology by Charles Lyell. This is a one volume abridgement of Lyell's classic, first published in 1830, which became a standard in the study of geology.
On the Origin of Species by Charles Darwin, 1859. This is a fascimile of the superior first edition.
Experiments in Plant-Hybridization by Gregor Mendel is the founding volume in the study of genetics. Published by Harvard University, this is the only edition of Mendel's work currently in print.
The Chemical History of a Candle is based on a popular science essay delivered by one of the great minds of the 19th century, Michael Faraday. He demontrates all of the known physical laws of the universe in the simple burning of a candle.