Koreader is an open source github project.
https://github.com/koreader/koreader/wiki/Download
Well supported in the forums here.
No, there is no IOS version to my knowledge.
But its supported in kindle, kobo devices and android, among others.
Try it out. you wont regret it.
Also i thought Onyx readers come preinstalled with their eink version of FBReader.