I love Pratchett's work. I own all of the books he has written or co-written, but have generally avoided the tie-ins that are largely the work of others. I should have avoided this one too.
The text, what there is of it, is almost entirely lifted from "Thud!" and is a "funny once" for those familiar with Discworld. And if you've read "Thud!" you've read this. But it's the illustrations which ruin this work for me. Pratchett has tended to avoid detailed physical descriptions of his characters, leaving it to the reader's imagination. I therefore found it jolting that the depiction of Sam Vimes, im particular, was so completely unlike my own mental image of him. Paul Kidby's illustrations in "The Last Hero" seemed to fit much better.
If you're buying this book thinking it would be good to read to a young child, think again. It is a book about a children's book, not the book itself. The illustrations are lavish, and it would appear that there's lots of detail to savor, but the depiction of Sam Vimes as looking like Buzz Lightyear really was a turnoff to me.
If you have to own "everything Pratchett", then go ahead and buy this too. It's certainly not bad. But I found it rather unfulfilling. If you haven't already, buy Pratchett's "Once More (With Footnotes)" from Amazon. Now THAT is a worthwhile read!