I am easily distracted while reading long works. That doesn't stop me from reading long comment threads, forum topics, or blog post after blog post, though. As best I can tell, the relevant difference is that the latter are broken up into small, easily processed chunks. Because I know I'll be done with the chunk I'm reading soon, my mind doesn't have reason to wander off. The commitment doesn't feel as large, so I have less reason to subconsciously question it. Just like with food, just one more small chunk won't matter....
My book-reading strategy lately is to discretize the material and be actively involved in the text. I scan each page, and if it looks interesting, I read it. I then try to write a couple lines of notes on the page or section I am considering. Focusing on what is worth writing down also keeps me from wandering off.
This strategy probably wouldn't work as well for fiction, but that usually requires less motivation. I hope my personal observations are of some help.
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.
Dale McGowan writes:
In this spirit, I propose the creation of a fully off-topic discussion thread.
Here is our monthly place to discuss topics entirely unrelated to Less Wrong that (of course) have not appeared in recent posts.
ETA: There are two behaviors I would love to see associated with this thread. First of all, discussions often drift off-topic in the middle of a thread. In these cases "let's take this to the off-topic thread" would be an excellent response. Secondly, given who's doing the discussing, I could easily see, say, a discussion about recent developments in some webcomic blossoming into a LW-worthy insight, in which case someone could spawn a new thread.