Admittedly, this is quite a short book: 103 pages, with illustrations, and smaller pages than usual. However, lots of shops are selling it cheaply: the RRP is £6.99, but I bought this at Sainsbury's for £3.49, and £1.61 of that goes to charity, so I think it's worth the money.
More generally, I've been pondering what type of genre this is. It's not exactly metafiction; that's when characters are aware that they're fictional, e.g. "The Last Action Hero" (a vastly underrated film). This is more the opposite direction, since it's the fiction of a fictional world: "fiction once removed", perhaps. Marvel did something similar a few years ago, when they printed "Marvel's comics", the conceit being that these were the comics which were published in the Marvel Universe. They included the familiar superheroes, but the MU writers wouldn't know about secret identities, so they went off in a different direction. "Itchy and Scratchy" (in "The Simpsons") is another example, but I can't think of any others offhand. I think you need a fairly strong concept for the first level of fiction, otherwise you get into Bagpuss territory where the first level is just a framing sequence for the main story, but it can work out well, as in this case.