I've tried most everything out there and given the answers suggested here and in other threads:
epub.js is easy to get to work but sparsely documented and does not really look to be under serious development. I couldn't customize it to my needs.
Readium is fairly hard to get to work. I got it to build just now and I'm trying to hook it into my epub system which is probably going to work but is more work than expected.
Update: I've now spent hours to try to get Readium to work and the project is too complex and the documentation too lacking to get anywhere.