I don't remember if you have Palm Doc format (not the eReader one), but that seems to be readable on most desktops or pdas/smartphones, at least Win Mobile and PalmOS. (It's the one called "Doc" at memoware...
http://www.memoware.com/mw.cgi/?screen=help_format) And those platforms are the only ones I'm aware of with iSilo support.
Oops... seems iSilo is supported on Smartphone, Symbian UIQ, and Symbian Series 60 also. But some Symbian at least seems to support PalmDoc...
http://www.memoware.com/mw.cgi/?screen=help_other
So as much as I love iSilo, especially with iSiloX for extracting web content, I'm thinking that PalmDoc has the more widespread support for people wanting to read it.
PalmDoc may also be what you are calling eReader/PalmDoc, I'm not sure. But if it's the same, you probably don't need iSilo also. On the other hand, for existing books, does it hurt to have an extra format?
Sorry to be so wordy, but hopefully this helps...