Originally Posted by fjtorres
If you go back to the bookshelf and long- press the center button you'll get a popup menu. If it offers an option to "open with" you can choose fbreader for drm-free ebook files and some document files.
Just so people are not confused if the quoted procedure does not work.
The standard key mapping in the 60x/90x have no function mapped to the long press on OK. The feature can be reached through the menu button (right side with an open book as icon).