I didn't specify, but the prices I'm looking at are for the ebook editions, so the binding isn't likely to be a factor. On reflection, it's less a case of book 2 costing more than of book 1 costing less -- the subsequent books cost about the same as book 2 (at least until one gets to book 5, which I see now is labelled "the first full-length novel"). I'd guess what it is is that book 1 was discounted to encourage people to try it out.