Take Garfield, photoshop out every character but John and the resulting consists of Jon talking to himself (or not talking at all), then any sudden change in behavior appears to be due to Jon thinking or talking to an imaginary person rather than a reaction to someone interacting with him. It's funny, fascinating, sad and inspired all at once. Garfield Minus Garfield
Ballantine Books released a compilation of strips in Oct. 2008. It was dedicated to Garfield creator Jim Davis with a foreword by Dan Walsh.