Sextus Empiricus, Outlines of Pyrrhonism. Sextus was a physician in ancient times. He recommended skepticism as a way of attaining peace of mind (ataraxia), and his book was meant as a guide. A useful summary of his views can be found also on Wikipedia.
Peter Unger, Ignorance: A Case for Skepticism (Oxford, 1979) provides more contemporary arguments in favor of skepticism.