As a general rule I tend to avoid books that are parts of long series. As a general rule, any series that extends into the double digits achieves its longevity by dumbing itself down, repeating old ideas, or just generally overstaying its welcome. However, the Discworld series has not yet fallen prey to this dreaded syndrome. Over 30 books into the series, Pratchett's irreverent satire, likable characters, and taut plotting still haven't begun flagging. Part of the reason is Pratchett's everpresent but rarely overbearing sense of humor. He manages to elicit laughter without ever cheapening the characters or devaluing the plotline.
Pyramids, one of the earlier books in the series, is the only one thus far that deals with none of the Disc's established characters (except Death, in a very brief cameo appearance). The main character is Teppic, a young man who graduates from the Assassin's Academy only to learn that he has a more pressing destiny: King of Djelibeybi, a thinly veiled version of ancient Egypt.
Interestingly, the book deals with many of the same topics as my previous entry, Watership Down. Political systems, religion, mythology, and death all play major thematic roles, but the perspective is entirely different. Rather than a serious examination of the themes, it is instead a deconstruction of them. The jabs range from obvious to subtle, sometimes making room for insights that, if they are not exactly profound, are at least thought-provoking. That, in a nutshell, is why Pratchett's work stands up and will continue to do so.