I have the same problem with finding it much harder to concentrate while reading an actual book as opposed to forum posts etc. I used to have a huge attention span for books of any sort when I was younger, so I wonder if being used to the short content you tend to find on the internet is the issue.

My trick is to go somewhere away from my house to read. Weather permitting, I walk to a park bench about 10 minutes from where I live. It makes it much easier to focus when I know I can't check my email or reddit almost instantly. Although it is annoying when I want to, say, look up the definition of a word.