Can/May I post a picture of something if I took the picture?
 

For example, if I have a pattern - and I took a picture of it - can i post that picture?

of a book?

From my experience I would have to say no you cannot.

taking a picture of a copyright item is the same as copying it and then publishing it. it cannot be posted here.

If I wanted to sell a book or a pattern, how would I display it? And I was unable to find a link to it -

Only the cover would be photographed or scanned.

if you had been clearer about your intentions, you would have gotten a better answer. yes, if you are offering something for sale here, you may take a picture of the parts that would be clearly visible in a store setting and post them here.

I have posted pictures of book covers/ pattern cover pictures to indicate where I got my inspiration from so others could search for it with no problem because there are no instructions included in the picture. The one time I did have a bit of instruction in a photo it was removed and I understand why.