Here's a clip from an article in Publishing Perspectives that offers some useful insight (where it says publishers, think author instead):
Using Twitter well is a skill. Publishers won’t get far by tweeting, “our book is great, buy it here!” From our experience, a direct plea results in a click-through rate of about 0.1%. Furthermore, such messages are not retweeted.
A smarter strategy is to share a compelling quote or fact from a book, one that your followers might share with their friends, who might share it in turn. A particularly poignant passage will have a much greater reach than a simple pitch for the title. The cascading effect of retweets can expose an exponentially larger audience to a book.
It also follows the golden rule of fiction workshops: show, don’t tell. If your book is great, prove it with material.
The entire article is Lessons from the Rick Moody Twitter Project