Graphic novels, particularly smaller format ones like manga. 6" is just about the right size for this. Larger, A4-sized graphic novels and comics can still be read, just in landscape mode or with panning. Zooming and panning can be a pain, but I find the 6" form factor preferable to lugging around a 9" or 10" LCD tablet with a third the battery life.

And also non-fiction works with photos or colour diagrams. History books or biographies would work, as well as technical reference books that use colours in their diagrams (I have the odd one or two loaded on my ereader).