The way I learned ePub was with Liz Castro’s book EPUB: Straight to the Point, it’s now probably out of it’s prime, as it only focuses on ePub 2.0, but I think it does a good job of explaining the ePub process from InDesign. I believe she also has a Fixed Layout mini-guide that would probably be helpful for you as well. ePubSecrets.com is a good blog to follow, if you’re on twitter, the #eprdctn hashtag is invaluble and there are a lot of people who post on there that can answer questions you may have. Colleen Cunningham (@bookdesigngirl on twitter) also put together a good resource page on Digital Book World’s site https://www.digitalbookworld.com/resources-going-from-indesign-to-ebook/. And if you have a chance to go to PEPCON, I would go, it’s a really good experience, especially if you are looking to get into ePub development.
Anyway, I’m sure there are also many other things to point you to, but that’s just what I came up with off the top of my head.