One of the reasons I've always liked Bandcamp as a consumer is that they made it simple for me to decide how I want consume the music. Maybe for this band I want to download a FLAC, and for this band I only care about an MP3. For less opinionated users they've long defaulted to MP3 which just about anyone can use.

You seem far too opinionated about your tech stack here to truly be a "Bandcamp" from the consumer side. You should try to meet customers where they are: if a user wants and asks for EPUB or MOBI (or MOBI with an AZW file extension) or CBZ or PDF or even DOCX for that matter, how good is your tech stack if you can't deliver on that choice?

A lot of people have favorite e-readers or e-reading apps, just because you offline-first HTML now doesn't mean it's necessarily the right fit. Especially in the world where users largely lost control of fonts and styling on the web, reader apps are people's best friends for long form reading.

It's technically neat that you plan to host arbitrary HTML to allow for interactivity in books. It's also an interesting security hurdle (your only plan to sandbox what you host is IFRAMEs?). But more importantly, if it gets in the way of being able to easily convert your hosted books to any e-reader format under the sun that a user may wish to pay for, is it doing you any good?

On the flipside you also don't seem to quite have nailed what made Bandcamp important to bands, either, which was its focus on bands. That early focus on bandname.bandcamp.com and being a "homepage" for a band listing their stuff alone without cross-sells, without being a "cluttered CD store", I think was critical to the early success of Bandcamp, and still a differentiator even as they've added a more somewhat unified catalog experience to the main page.

Where's your focus on authors? Again, your tech stack doesn't help here: a lot of writers use Word, your dripping disgust of Word in your FAQ shows that you don't seem to care about the tools that writers use. (A lot of professional writers use Word at least somewhere in their toolset because a lot of professional editors rely on the proofing and editing capabilities of Word.)

You have an ordinary bookstore homepage with an admixture of writers with no real organization and a focus on books over authors. You've already let it be a dumping ground of public domain authors, diluting any possible message that you are author-focused. Your author pages are okay, but they don't feel like a "homepage" for the user, partly because the URLs are ugly presumably SEO focused things (nickname-firstname-lastname; that's a lot of things), rather than author-focused personal brand choices. On top of that, your footer is a full third of the page of an Author with only a single book, making the page much more about you than them, and you've filled the page with your logos and your widgets such as login/signup, and including a floating "Need help?" button that also seems to be more about you than about the author, and another location for you to include your logo. Have you ever noticed how little Bandcamp brands itself on a band's page? The overall pixel count is pretty low, the number of widgets kept to a minimum (and pretty artist/album specific: buy, wishlist), used to be typically only a single tiny logo, and again the focus has always been on the artists.

If you are going to compare yourself to Bandcamp, it might be a good idea to get a better idea of what Bandcamp does right, and how they operate as a platform.