Isilo is a proprietary format, and can't be converted to anything else - see this thread:
https://www.mobileread.com/forums/showthread.php?t=75338
There are no eInk devices which support the format (it's obsolete), but there is, it seems, an isilo app for Android tablets.
Perhaps a good idea to start buying books in a modern convertible format like ePub. I imagine your books must be pretty ancient to be in isilo format?