Okay, yup, slightly different header expected, and it seems to work.
Dirty patch attached. I have yet to check that I didn't break anything for: Non KF8 files, other devices/FW. So, take with a grain of salt.
Also, in the APNX I checked (from Neil Gaiman's Stories), there was a second pageMap tuple & possibly a second Page List data blob. No idea what it's here for.
EDIT: Well, I even have some files with 3, 4 of 5 tuples. Yaaaay!
Found files with the extended header both on my Touch & K3 @ 3.4, both for KF8 files and mobi6, so, hopefully, the patch doesn't break anything, at least on up to date devices...
EDITē: Updated the comments in the patch accordingly.
@kovid: I have a small bunch of minor KF8/Amazon related patches (laying around in my Portage overlay)
, how would you prefer me to submit them: in a thread here in the dev subforum, or over on launchpad?
EDIT^3: Killed deprecated attachments. Fix merged upstream.