Twitter isn't built to sell books. Or anything else.
Yet if you belong to the Twitter, you see all sorts of authors pimping their books.
Some do it subtly, or randomly. Others do it faithfully, if not relentlessly.
And even if they mount a full Social Networking Offensive -- a combined-forced attack with tweets on the ground, blog battleships at sea and Facebook fighter planes swooping down from above -- even if they do all that, they will fail.