He is the one author I never weary of, and have read his complete series several times--and each time I do, as I age, I see more, and in a different way than I did before. Every time, it's different. He is funny, and sad, and can move me to tears on page 17 and have me laughing out loud two paragraphs later.
He makes you think, and rethink, all the while teaching you things. He has subtly delineated the history of the British Postal system, printing, money, (and printing money), how the British policemen and their place in London came to be. The Buddhist monks come into it, dragons, strange gods, wizards, and talking dogs; as does his philosphy of death, life, time, and humanity.
You end up caring, and thinking, about his characters.
He has written (besides a children's series and myriad of other things) a series of DiscWorld books, 41 strong. He has been compared to Douglas Adams, and yet when I read Adams, I am rarely moved, and often disappoiinted, because he seems to be a one note cynical writer.
I have never seen a single book of Pratchett's in the used books bins anywhere, from yard sales, church book sales, or thrift store shelves.
Anyone who has never read his books, or wants to know where to start, start at the beginning, but don't be discouraged by that first book (The Color of Magic). It's not that good, but it introduces you to the characters in embryonic form. I only read it now because it is the first book, and the ones to follow only get better, and it's nice to see how his gifts as a writer grew.
It's obvious Pratchett loves his characters, even the despicable ones. And I am reminded not only of Dickens but Shakespeare, both of whom wrote across the spectrum of humanity by the time they were done. Each of them wrote for the time they were in, as does Pratchett.
There. Now I feel better =)