In 1665, Samuel Pepys recalls in his diary that he stayed up till 2am one morning, reading a best-selling page-turner, which he called "the most ingenious book I read in my life". It wasn't a book about history, or a play, or anything from the arts: it was the world's first popular book about microscopic images.
Titled Micrographia: Or Some Physiological Descriptions of Minute Bodies Made With Magnifying Glasses With Observations and Inquiries Thereupon -- catchy -- it was written by famed scientists Robert Hooke. Now, it's available on Google Books for you to read, and it doesn't cost a cent.
It's well worth taking a flip through: there are some wonderful drawings in there, of everything from fleas through bark to the edges of razors, and it's exciting to see what constituted cutting-edge scientific understanding all the way back in 1665. If you're interested, Carl Zimmer has written a wonderful review to whet your appetite further. [Googe Books via Download the Universe]