Our customers could not care less about the design stuff as long as it looks decent, so it is not something I have bothered with even now that I have some money I could spend on it.
I image cross-platform compatibility is a PITA.
But I find it interesting that people who pay for ebook design, don't care about website design.
Mind you: That's no criticism of you OR your work.
I just love the fact that people are more interested in the value than in the website design, even if they get value out of design work.
Hope that makes things clearer. Again: You're doing a great job!
eBooks are mostly about stripping out people's "design" stuff and replacing it with some fairly standard components that actually work, and adding in some various bits and pieces in XML, a hyperlinked table of contents, and stuff like that.
I like that you kept the layout simple using only bootstrap. that's a lot faster & cheaper than doing a full blown design.