Dragons at Crumbling Castle: And Other Stories

Not his best work, but it is obvious how Pterry got from here to the vast heights of his later works. Early versions of The Carpet People and prototypes of some of the characters and situations that would crop up on the Discworld* are all here, just lacking that final flourish. This might actually make a pretty good introduction to the author for younger children.

*Not to mention plenty of footnotes!