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]