I have a couple of observations after also getting a few books that wouldn't give me the covers.
1. Do NOT check off "Enable sharing of book contents via facebook etc..." when converting to .mobi, or you will most certainly end up with the "Personal" banner.
2. I finally figured out that the books I was having trouble with were the ones for which I had downloaded metadata from sources other than Amazon. So here's what I did to solve this issue:
a) Edit the book's metadata in Calibre.
b) Click the "Configure download metadata" button, and choose ONLY to download metadata from Amazon under "Metadata Sources." Save your changes by clicking the "Apply" button.
c) Click the little Delete icon next to the "IDs" field in the main "Edit Metadata" window. This clears all the existing values from that field.
d) Click the "Download metadata" button, and save whatever metadata is pulled back from Amazon. Note that you do NOT need to save the cover that comes back from Amazon. What cover is in Calibre doesn't make a difference for "Cover view" books that don't have the "Personal" banner. (i.e. the "non personal docs" covers are pulled down from Amazon).
e) Try downloading the book onto your Paperwhite via the network. If all is well, then the cover should show up in "Cover view" with no "Personal" banner.
If it doesn't show up, try these things:
a) Make sure that before you search for metadata on Amazon's site, the title of your book is an EXACT match for the title of the Kindle book as it exists on the Amazon web site. If you're getting generic covers, then this is something to check. If the book doesn't exist as a Kindle book on the site, then as far as I can tell, there is no way to get a cover to show without the "Personal" banner attached, as "Cover view" on the Paperwhite pulls the cover images from Amazon based on the ASIN number (I noticed this because the cover that shows up in "Cover view" is sometimes different from the one I have in my Calibre metadata). Your only option then is to convert to .mobi with the "Enable sharing of book contents via Facebook" option selected, which will put the Personal banner over the cover.
b) If you've tried all the steps above for a book that exists in a Kindle edition on the Amazon site, and the covers still aren't there, then install the Quality Check third-party plugin, and right-click your book in Calibre's file list, and select Quality Check > Fix > Fix ASIN for Kindle Fire. (Link to Quality Check plugin: http://www.mobileread.com/forums/sho...d.php?t=125428
Hope this helps! I've now got about 95% of my covers showing up without the Personal banner -- all except those that don't exist as Kindle editions on Amazon's site.
Man, they really make you jump through some hoops for something so seemingly simple.