HI,
I don't know if you've already solved your problem but I found a converter that turns text files into PDB files, and vice versa. I read ebooks with a Palm i705 handheld. I'm saving for a Sony Pocket, but when I get it, the Palm will read the files the Sony can't. Anyhoo, maybe you can try that program. It's converted most of them so far. As for HTML, I just convert them to text with Word, and then convert them to PDB. Good luck!