May I post a picture of?
 

If I buy a jelly roll - or a ruler - or a book - or whatever - and I take a picture of it -

would it be infringing any copyrights if I posted that picture?

for the most-part ... no - you would not be infringing.
you own the item.
you took the photo.

when it comes to the book you have to be more careful.
if the photo(s) are clear enough, and of enough pages that the book itself is not needed then that would be a violation.

If I just post a picture of the front and back covers of a book - say enough so that someone else can identify it - would that be okay - if it's my book and I take the picture?

the way I look at it... if it is a picture of what would be visible on the store shelf, it is OK.