I like to think that some of our books are between 30% and 110% of what you'd expect from an O'Reilly book. They don't have things like indexes, but the advantage is that authors can write in a dialect of Markdown, preview or publish with one click, and distribute updates to readers automatically.
So, many of our books start as 30% or less of what you'd expect from an O'Reilly book, and grow to be as much or more than one.
In terms of the "or more" part, this can include being more specialized than what would be considered marketable.
It can also, however, include being offered as a course: a Leanpub manuscript written in Markua (one of our two Markdown dialects) can also be used as the basis for a course.
To learn more about that, the spec is here: https://leanpub.com/markua/read#leanpub-auto-creating-a-cour...
(And thanks to that bug being pointed out to us, that link should even work well on phones now :)