There are a couple of convertors that should work for a lot of file types.

1) PDA Convertor - it will convert a plethora of formats to Plucker which can then be read using FBReader.

2) Sunrise (0.42j if your desktop is Linux/Mac or SunriseXP for windows) - is great for parsing large html files with links and images to Plucker format. I've used Sunrise to parse entire online text books. It also works well for parsing RSS feeds with complete articles for offline viewing.