Along with Kindle for Mac 1.7 and Kindle for PC 1.7, released a few days ago, Amazon have introduced a new ebook format for their Kindle ebook store. Called "Print Replica", it is Amazon's solution for books with a page layout that is too complex to reproduce using their usual Mobipocket-derived ebook format.
A "print replica" ebook resembles a PDF version of the book. The page layout is fixed, and the only viewing options are to zoom and to pan. There's no font resizing or page reflow available.
This may explain why the format can only be read with Kindle for PC or Kindle for Mac, as trying to read a fixed page size format on the relatively small screen of a Kindle is a sub-optimal reading experience, unless the fixed page size is the same as the Kindle screen. Amazon has not said whether current or future Kindles will support the format.
Another innovation introduced with this new format is the ability to rent ebooks rather than buying them. Books can typically be rented for between 30 to 360 days, although the exact limits will vary from book to book. And a rented book can be bought with full credit given for the rental fee, if bought before the end of the rental period.
Full details can be found in
Amazon's Help pages.
Investigations of the format via free samples shows that it not only resembles a PDF, internally it
is a PDF file, along with a few Amazon-specific extras that might be index entries, all wrapped in a Palm Database file.
[EDIT: Oh, and they seem to have the file name extension ".azw4". Thanks for the reminder, HarryT!]
[EDIT2: And under two weeks from the first appearance of this format, it's possible to strip the DRM (thanks to Apprentice Alf, et. al.), import into Calibre (thanks to Kovid Goyal and team) and extract the PDF (umm, me), through various tools from various groups. Amazing.]