The decline is fairly small on a percentage basis and the reasons are debated--although it's definitely happened. A fairly sharp increase in audiobooks seems to be one reason. I suspect that people have also figured out that eBooks are good for some materials and not for others. Once you get away from books that are mostly flowing plain text, fiction in particular, eBooks don't generally work nearly as well.
I haven't seen pricing given as a major factor. Used books certainly compete with the eBook back catalog but they aren't usually counted under physical book sales.
By effectively you mean steadily declining sales as people return to printed books.