If you don't mind removing DRM, then it's quite easy to convert to something the 505 can view. Takes maybe 5 minutes tops to go from Mobipocket or MS Reader to LRF for the 505. That's removing the DRM and using Calibre to convert.
To remove DRM from Mobipocket, you want MobiDeDRM.py 0.04 or 0.05
To remove DRM from MS Reader, you want ConvertLIT 1.8 and you'll need a Windows computer to be able to run ConvertLIT and purchase MS Reader format.
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