Gyms (fitness health clubs, whatever) are potentially a great analogy, in that like books, they are used a lot by a tiny fraction of their membership. But gyms mostly make money on the people who buy memberships they don't use, so maybe not such a great analogy after all. Certainly, they display a related unevenness.
ETA: I guess where I'm going with this is that a niche market can work for or against a general market. If the niche market attracts all the people who cost the general producer a lot of money without a lot of return, the niche market is _great_ for the general producer. But if the niche market attracts all the people whose margin was funding the general market, which without them is an overall money loser, then the niche market can potentially wipe out the general producer.
And books display this problem in a self-similar way -- everything about books has the value concentrated in very limited areas, that historically you've only been able to get at by going after large swathes, but the added information introduced by the internet has really changed things. The used market, for example, first wiped out most bricks and mortar used book stores. Initially, it created a flowering of online stuff, which is now being winnowed heavily.