Daniel replies:

So a few years back someone from Random House told me they were doing a paperback of Blue Moose and Return of the Moose. ""Why don't you include The Moosepire, and have a complete set?"" I asked. I wound up offering to let them have the third book at no charge, doing all new illustrations, writing a foreword, and/or a fourth moose story for free, doing a new cover, and paying them one thousand dollars, if they'd print The Moosepire with its companion pieces.
The book as they published it has Blue Moose and Return of the Moose, and not The Moosepire.
And that, in a general way, is why there is no I Snarked with a Zombie. Different publisher, same mentality.