Once, Yes, Once For a Lark. Twice, Though, Loses the Spark.
The series, the sequel, and how they stand in the world of "real" literature is the topic of a thoughtful posting at Oz and Ends. Does a sequel, after all, have any merit if you find you can't consider it without looking at its predecessors? I mean, The High King and The Grey King both won Newberys and they weren't exactly stand alone. Or were they? Certainly it's worth thinking and chewing over. How much credit do you give a book that's intended to be the first in a series vs. a book that becomes so popular that the author eventually MAKES it into a series? Oz and Ends considers the ramifications.