I use my N810 almost exclusively as an ebook reader (FBreader is amazing!!).
To reduce eyestrain you might find helpful a small app called advanced backlight. It gives you access to the full 128 levels of backlight, as opposed to the 5 levels that Nokia provides, so you can set the brightness very very low and a dimmer screen really help reduce eyestrain.
Another app I use all the time is Stardict dictionary, it has the ability to automatically throw up a definition whenever you highlight something in a program running on the Nokia (FBReader, browser, etc).
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