There are two worthwhile things about this book: Firstly, the introduction by author A.S. Byatt, which is superb and really, for once, properly analyses Pratchett’s style, approach to writing, and what makes him unique. From this I learned about the concept of “secondary worlds” in fiction. Secondly, there are Pratchett’s collected short stories and other writings, some with “Discworld”-like characters, others like Discworld precursors, and others yet completely different.
What is really interesting is that Pratchett seems to have started writing with his style developed quite far and his ability to create imaginary worlds already highly formed.
The first story, published when he was 15, clearly shows the brilliance which would be refined in later novels. His comments on how and why he wrote the stories, and whether he likes them, are very entertaining and explains how he created his novels and what motivated him. If you want to understand Pratchett and his work, read this.