I've successfully used my credits for Kindle books even though it didn't show up under Kindle editions for me either (just music and video) so I wouldn't worry about that too much. I also returned a book purchased with my credit (ended up cheaper elsewhere) and got the credit back. I did have one problem where one of my credits didn't show up in my account right away even though I got an email about it. I contacted Amazon via email to ask about it and they gave me a $5 credit to apply to anything, then the credit showed up, and when I contacted them again they told me to just keep the $5 plus my earned credits.
