Originally Posted by Skydog
To change the tag to [EBOK] : Preferences => Conversion => Output Options => Mobi Output => Personal Doc Tag = change [PDOC] to [EBOK].
I did this and it worked.
I just re-converted my book using Calibre and changed the tag as above.
Then I deleted the problem book from my kindle and from my amazon manage my kindle" page, and resent the newly formatted book to my kindle via email.
I also had deleted it from my iphone app and actually removed the app from my iphone just to make sure. I reinstalled the app and the newly formatted book appeared in my cloud section. I opened it and it correctly opened on the last page read from my kindle.
Now my kindle and my iphone app send last page data back and forth perfectly.
Note that in my case only a few books were giving me trouble. So in my case it seemed the files were at fault.