Novella-length works are becoming viable again, thanks to ebooks, audio books, etc. They are also popular as supplemental works for long series. Granted, this info comes from a genre panel, but the theory stands.
Part of the issue with eBook sales is the uneven pricing between ebooks of similar length, and the persistent attitude among consumers that a book without an attendant physical artifact is is simply worth "less". As the ebook-as-platform matures there will likely be convergence among different producers, distributors, etc, which will increase consumer confidence.
eBooks users can be stuck in the middle between two less-than-optimal situations:
If an ebook distribution/platform combo dissolves (Kindle, Nook), users lose the thing they paid for (even though technically, you don't buy an eBook, you lease it)
It turns out that it is illegal to resell an eBook (because of the lease thing). You can resell e.g. a PDF or something without embedded DRM, but the thing itself, to the extent it exists, is the property of the distributor. This means there is no secondary market for ebooks.