Probably your answer will depend upon how / what application your users will be using to read on their device. Ereader now supports just about every mobile device platform including the iphone. So, putting your books into ereader format wouldn't be a bad choice. Plus their reading software is free, so the barrier to entry is low.
Some reading software does support reading html files, plain text files, and rtf, but I personally find that html files cause more problems than they solve on small devices. Just make sure that whatever format you choose is
1. Reflowable and resizable-- your user shouldn't have to scroll from side to side as well as up and down to read the text.
2. Has fonts that are adjustable in size-- Mobile device screens are of varying sizes and resolutions, so stay away from formats that lock the font to one size. My aging eyes find ridiculously large fonts a better option on small devices.
Unless you must control every aspect of the layout, pdf is generally a bad idea for small screens. I understand that there is now a version of pdf that will reflow, but it seems to remove any advantage of creating the document as a pdf to begin with.
It looks like you work for a publisher. If so, your options may simply be dependent upon whether your company has decided to use DRM or not. I think staying away from restrictions is always a good choice for your readers and brings goodwill to your company.