What work of Tacitus records the years 14-68 in 18 books?

The work of Tacitus that records the years 14-68 in 18 books is the "Annales." This historical account provides a detailed narrative of the Roman Empire during the reigns from Tiberius to Nero, effectively capturing significant political events, cultural dynamics, and the character of the era.

"Annales" is particularly important because it covers a critical period of transformation and turmoil in Rome, including the transition from the Julio-Claudian dynasty through considerable political unrest and change. The depth and analytical perspective found in "Annales" set it apart from Tacitus's other works, such as "Histories," which focuses on the Year of the Four Emperors and subsequent civil wars, and "Germania," which describes the customs and geography of the Germanic tribes without the same focus on Roman political history. The "Dialogus" serves as a philosophical dialogue about rhetoric and its decline in Rome, thus not fitting the criteria of covering the specified period in a narrative historical format.