Strangely enough, this exploration of one of the most private of writing endeavors is likely to send readers off in a zillion different directions. Thomas Mallon's survey of diarists throughout the ages introduces us to the most personal writings of more than 100 diarists, including Samuel Pepys, Leonardo da Vinci, Virginia Woolf, and Lee Harvey Oswald. Mallon divides the diarists into seven categories--chroniclers, travelers, pilgrims, creators, apologists, confessors, and prisoners--that he uses as a basis for his inquiries into the nature of these apparently private writings. (From the start Mallon admits that "I still don't believe that one can write to oneself for many words more than get used in a note tacked to the refrigerator saying 'Buy bread.' ") Glimpsing the many, vastly different lives that have been thrown together on these pages is fascinating in and of itself, but Mallon's thoughts about the whys and wherefores of diary-keeping are what make his dense prose so worth reading.