As I've been experimenting with producing EPUB files this week, I've come across a handful of really helpful resources.
Liz Castro's blog and guides
First and foremost have been Liz Castro's two publications, EPUB Straight to the Point and Fixed Layout EPUBs Miniguide. EPUB SttP includes a very helpful chapter on moving from InDesign to EPUB, but I found it to be incredibly helpful for the InDesign-free project I just completed. It's a fantastic orientation to the EPUB file structure and includes essential basic information from unzipping an EPUB (so you can see and tweak its guts) to recompiling (even if working in Terminal makes you nervous, her instructions are super explicit so you won't go wrong). Her blog is also a great source of information. One of my favorite aspects about her site and her publications is that she's very neutral about tools. Highly usable.
EBW Knowledge Base
EBW is a digital publishing consultancy and their EBW Knowledge Base is a great collection of short instructionals on creating ebooks. This includes a healthy InDesign-specific how-to section, a higher-level workflows for publishers section, as well as a section of general tips for self-publishers. Finally, their Other Resources page is a great, annotated roundup of production tools, readers, and further reading.
Liza Daly's talks
Liza Daly is a boardmember for the International Digital Publishing Forum and president of Threepress Consulting, which extends her impressive experience in developing digital publishing tools. She has published a number of her presentations (with narrations!) to the Threepress website. Several of her talks focus on the state of the industry and as such, aren't immediately instructional, but they provide a great orientation to the emerging technologies and standards defining digital publishing. Her Interactivity in EPUB talk provides a great discussion of what's currently supported.
Robert Nagel's braindump
Robert Nagel is a technical writer and independent publisher. He recently posted a massive roundup of his EPUB production tools and tips. He also catalogs common glitches and fixes. We should all create and share such "braindumps."