cover image has an id != "cover".

[dteviot]

Warning message from Calibre

The cover image has an id != "cover". Renaming to work around bug in Nook Color.
Refer #46

[dteviot]

Sigh. Still not right. Now I have two elements with id="cover".
I think I might try making the cover as indicated here:
https://www.safaribooksonline.com/blog/2009/11/20/best-practices-in-epub-cover-images/

Different instructions
http://wiki.mobileread.com/wiki/Ebook_Covers

May also be worth adding a cover-image property to the item, in anticipation of generating EPUB3.

<item properties="cover-image" id="ci" href="cover.svg" media-type="image/svg+xml" />

refer http://idpf.org/forum/topic-715

Based on above links.
href of the cover image file = {anything}.
ID of cover image = "cover_image"
href of XHTML wrapper for cover image = "cover.xhtml"
ID of XHTML wrapper = "cover"

metadata section gets content set to "id of image file"
guide gets href of xhtml.
spine gets id of xhtml.