Well, I want to give kudos to the publisher, Aquapress. I emailed them last night and when I woke up, I had a response (they're in the UK, and I'm in the US).
They were surprised and explained their quality control process and said they hadn't seen this issue before. They'll send me a replacement.
I'm less impressed by the online seller from Abebooks, from whom I received an autoreply that USPS tracking shows the book as received. No human response to my clarification email.
The strange thing is that the spine looks like it's designed for a book this thickness (1cm). My first thought was that maybe either the pages were there but displaced or there may have been duplication of other pages, but neither of those is the case.
In any case, all's well that ends well. It's nice to see a company staffed by human beings who actually communicate with customers and make good on any errors.