I could care less for a PDF - but epub is a pretty useful format for actually reading a book. And a Kindle book is also rather nice (mostly due to kindle syncin

I could care less for a PDF - but epub is a pretty useful format for actually reading a book. And a Kindle book is also rather nice (mostly due to kindle syncing location in the books across devices etc - I would prefer to buy ebooks without the DRM, as there's the worry one will lose access for various technical or business reasons). And I'd like an offline copy - I do sometimes read in places without Internet access. Without any of these - I'd be hard-pressed to buy - even if the content is excellent.