Apparently this is something of a meta-book, having been mentioned in the Discworld novel Snuff. The book brims with Terry Pratchett's characteristic humor, forthrightly brought to bear on a basic human need. It's not his funniest book, but it's light, good-natured, and fun. The illustrations are nice, if not as light-hearted as one might hope.
The plot and characterization are pretty simplistic, but that's not really what the book is about. It reads more as an off-the cuff in-joke that's also accessible to outsiders.
The World of Poo is a quick, easy read that's a fitting cap to Pratchett's long Discworld series (though it's been available in the UK for some years). If you liked his other books (and you should), you'll like this.