At the moment, most iLiaders are reading PDF files formatted in A5 size. On iRex's site you will find a download file containing instructions for converting your existing docs to this format.
I use Word and PDF Creator to convert from RTF but there are many other programs that you can use.
HTML files will display on the device but they don't flow well and often cut off the top and bottom lines. As for RTF I haven't tried -- but I should imagine these will suffer the same problem.
However, we are promised that future software releases will address these issues and I am looking forward to the day when I can read my RTF and HTML files without conversion.
By the way:
http://manybooks.net//
has free books specially created for the iLiad.