The Rama Series:
Rendevous with Rama (by Arthur C. Clarke)
Rama II (by Arthur C. Clarke & Gentry Lee)
Garden of Rama (by Arthur C. Clarke & Gentry Lee)
Rama Revealed (by Arthur C. Clarke & Gentry Lee)
Bright Messengers (by Gentry Lee)
Double Full Moon Night (by Gentry Lee)
IMHO, this is the greatest novel series of all time, especially in the science fiction genre. Never has a set of novels so deeply explored humanity. For those of you that have read any of the series, what are your thoughts?