The circle design will fail with huge books or collection like Steven Erikson's
Complete Malazan Book of the Fallen or Robert Jordan's
Wheel of Time, which are in the neighborhood of 3.5 and 4.4 million words respectively. Luckily, e-books like these aren't very common. Kobo could maybe pick the fuzzy route and simply state that these e-books have more books than the average book. I still don't like it.
Well, I guess there's no one-size-fits-all approach, right? Would be nice if Kobo made some things configurable but that's off the table, I think.