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.
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.