Well, the works are autobiographical, as well as incoporating the geography and ethnic background of Britain.
Tacitus� style is peculiar, full of imbalance, innovative exploitation of traditional grammar and syntax, and Tacitus� personal commentary on the state of Rome/Roman politics and the individuals who people his works. Tacitus in general paints a very negative picture of the principate (as opposed to the Roman Republic) and is often more concerned with pointing out the problems that he thinks are inherent to Rome�s imperial form of government than he is with relating history objectively or straightforwardly. In many respects to understand his work you need to understand the man, and Tacitus was very bitter towards the upper social classes because of the level of corruption he experienced first hand. Some info here ....
http://en.wikipedia.org/wiki/Agricola_%28book% 29