Jump over to
this read in the Sony Dev forum - Morkl has done some fabulous work modding the original reader app, including fixing the bold/italic problem when selecting alternate fonts and being able to change fonts.
You'll need to wade through the thread to pick up the latest version (post #39 has the current version), and it's probably not what you'd call polished yet (he's only been at it a couple of days and you'll need to hand-edit some xml files to take advantage of all his changes) but he's made great progress so far. You'll need to install the mod (which isn't too difficult) so this won't be for everyone though.
Cheers,
Simon.