It turns out that Amazon release dates are not as smooth and seamless as one might expect. Today is in fact the 27th, but the page still says to pre-order the book. (Because, you see, it comes out on the 27th. Which is today.)
I shall factor this in to planning for future books.
At least the customer service people were great! They're working on the issue ASAP, and it's likely due to a delay in unpacking boxes at the warehouse, or something of that sort. Hopefully everything will be resolved soon. Hopefully!