I think you could sell ebooks if you were willing to part with 30% of the revenue. Physical books that come with ebook access, or DVD/Blu-Ray that comes with digital access are treated as physical purchases, I believe.

My guess would be that the rule is that there's no Apple cut if you're selling a physical thing and there's a digital attachment. But Apple probably reserves the right to disregard sham physical transactions, where the physical item is de minimis in value compared to the digital item.