See
Wiki: PRS600. It does not support either LIT or HTML, but both can be converted to ePub or LRF for the Sony using Calibre (has its own forum here). If the LIT has DRM (encryption) you would first need to circumvent this and then run Calibre. If you are buying ebooks, then look for Adobe ePub (or PDF or "Digital Editions") ebooks, which are readable without modification on the PRS-600.