Hmmm... Well, I think it depends on the book, at least partially. When I read Dante's Inferno, I had absolutely no idea what's happening
. Labyrinth of the World and Paradise of the Heart, by John Amos Comenius, was written in really old language, but I really liked it, for it was full of edification. Stendhal's The Red and the Black, that was pain in the neck. Really, five hundred pages that were so boring, gave me nothing, and the story could be summed up well in two paragraphs. I love ancient stuff, like Gilgamesh, Nibelungs, even Illiad, even when it's written completely seriously I find it so hillarious. Illiad had six hundred pages, I still remember it, along with some good lines. This is thing of taste, but that's personal matter, so not what you're searchin' for. Well...
What helps me with keeping concentrated while reading, is to project the sentences in my head. We know that people remember stuff better when they read it out loud. I think it doesn't need to be read out loud, I am like recollecting, repeating the words in my head, just after I saw them. It's like reading out loud, but in the spirit. I do this when I am praying alone with the breviary, it saves me time, yet I concentrate on the prayer. But again, if something's so bad and hard to read (you don't agree with the stuff or whatever), it's pain in the neck to do this, and it's inefficient anyway.
May I ask you question? How often do you read?