Typically, political action groups, think tanks and other politically affiliated organisations will political books in bulk and then distribute them to its members and prospective members in exchange for subscriptions, donations and the like. It happens on the Liberal side of publishing, but conservative books are by far bigger sellers.
Read the full article here...
As the article says, in no way is anyone doing anything remotely illegal. It is a bit of a stretch to boast about this books sales or that one when outside organisations are buying them up hundred at a time. Levin's hardly the only one to benefit from the practice. Sarah Palin, Bill O'Reilly, Sean Hannity, etc....all have seem similar arrangements I suspect.
All are popular figures and you'd expect them to have strong sales given their audiences. Its my opinion that political books bought in bulk should not be considered in the regular sales figures. Its a bit misleading to the average consumer.