From Cave Paintings to the Internet A Chronological and Thematic Database on the History of Information and Media The First "Computer Manual" (1606)

In 1599 Galileo Galilei developed his geometric and military compass into a general-purpose mechanical analog calculator, later known in English as the sector. 

As an instruction manual for purchasers of the compass, and to establish his priority for the invention, in 1606 Galileo published Le Operazioni del Compasso Geometrico et Militare in an edition of only sixty copies. To avoid having the compass pirated, Galileo had no illustrations of the device included in the pamphlet, which may be considered the first "computer manual."

During the seventeenth century the sector became one of the most widely used mechanical calculators for scientific purposes.

You may view a digital copy of Galileo's Compasso at this link.