As you've discovered, reliable info is hard to find — maybe harder than you realize, since a lot of very plausible, seemingly quality resources are either horribly out of date or take very idiosyncratic approaches, but like biting into a bad egg, you don't really know that until you're there. (Always look for dates on posts and tutorials, if there even are any. Two years is getting feeble and probably irrelevant to current practices.)
The e-book world has yet to quite mature, and not only are there many different 'factions' in the game, but most are opposed if not hostile, and — worst of all — no one ever takes down or updates old material. (Don't be surprised if you find dates six to ten years back... which is completely obsolete.) There are very shiny, easy, cheerful tutorial/how-tos out there that may as well begin by telling you to stoke up your steam boiler. (I kid but little.)
But the stable, professional approach is easily doable now, which is good.
There are two phases to producing quality, robust EPUBs from InDesign. One is all the basics, and courses such as Derek suggest are a good start on those, especially since you indicate you're a training course kind of user. (Not everyone is; I'm in the "hand me the manual and leave me alone" camp on most things. 🙂 )
But most courses take a very generic, vanilla, simple approach to books and don't get into the real complexities of EPUB and Kindle; like many of the online gurus who boast they've done 500 books, it and they stop at fairly simple novel/flowing narrative content. Which is barely even Step 1 unless you're going to do nothing else.
For textbooks, reference manuals, any anything with a more complex layout (images, tables, foot and end notes, etc.), there are not many solid references. Well... other than mine. 🙂 And I'm always happy to answer specific questions or help work through EPUB development kinks.
If you have all the related experience (e.g. you aren't new to either ID or book layout) you should be able to master the basics in a few days and turn any of your titles on hand into a reasonably clean EPUB. ETA: you may find this post useful.
What kind of books are you generally looking to produce, for this client and others?