I think is good idea to add guidance line help for easier the reading experience. This is especially good in the following cases:

1)For people with sight problems. I recently helped to install a MagniLink X Reader [1] on my local public library and it has a guiding line you can move on te screen.

2)When you are reading a text on the train for example. I realized of that the week before, I carried my EEE PC with me on the train and I wanted to read some documents, but I lost the line I was reading every time :)

We can implement this with a guiding line or selecting the line text.

[1] http://www.lvi.se/CM.php?PageID=42461