I have read only three: Trials of Dedan Kimaathi, Petals of Blood and Wizard of the Crow, but through them I took flights to Kenya and got my first language and history lessons about the Kenyan society.
Petals of Blood had a great effect on me (not until I read Wizard of the Crow) but the lasting one is the fact humans have the capacity for change. People who desire to change the world should master their minds first; and that hunger can reduce an human being to anything if s/he does not master his/her mind. For me, this answers questions of accountability, corruption and sincerity to the people (among other issues) that poiliticians face when they are voted into power. And I am not surprised when the seeming good person starts turning out as bad (forgive my words!), when the revolutionary activist-turned-politician is suddenly guilty of offences he was raining fire and brimstone against. Overall, his works are Kenyan and African and touches on personal and political issues that everyone should know about!
Ngugi is a master storyteller; well written works with a motive: to shake you out of your 'comfort' seats and react by asking for change or being that change.
He has been nominated for the Man Booker prize, 2009. Click here for more information.