Welcome to MobileRead, DrSiddons!
The formats that work on the Sony Reader are TXT, RTF, PDF (only works well if the page size is the same measurements as the Reader's display), and LRF/LRX/LRS (Sony's proprietary BBeB format). There are, however, a number of tools discussed around the forum that will convert unlocked PDB/PRC/eReader files into RTF.
