1. Sell, Sell, Sell
Would you ever return to a store where once you walked in the room someone started yelling at you: "Buy this! Buy this! Buy this! Buy this! Buy this! Buy this!" Probably not, so why would you ever use Twitter that way? If every tweet you post is "Buy my book!" people are not going to buy your book. Even if 50% of your tweets are like that you are still going to turn off a lot of people. A good rule of thumb is 80/20, 80% non-selling, 20% selling or other promotion (such as follow my FB page, or look at my blog tweets).
2. The Forever Tweet
There's a reason it's limited to 140 characters. Do Not post multiple tweets on the same topic with no end in sight. It's okay to carry on a conversation with others but don't just fill your page with a long string of endless tweets on one subject.
This especially doesn't appeal to followers because tweets go in last post order, meaning the first thing posted is at the bottom, so they have to scroll for a long time to see your original thought and then scroll forward to read the entirety of your ridiculously long rant. Imagine if you had to watch a TV show in reverse, it would be annoying just like a never-ending tweet rant.
3. Not Interacting With Others
While the other two are about what you might be doing that turns followers away, this is what you might not be doing which isn't allowing you to attract followers. When you choose to follow someone it will work to your advantage to re-tweet something they've said, star/favorite something they've said or tweet directly to them with some sort of question or comment (not a direct message, that's different and typically frowned upon). By interacting with others you're directing them to your page and they'll usually follow you back (if your tweets are interesting to them and not examples of 1 or 2 above). Now some people are too busy to follow everyone that follows them and nudges them with a RT or a star but you'll get more followers by interacting with others. Also jumping into a conversation or responding to others is a great way to get noticed and if you're polite (not attacking others online) then you can typically get some followers by joining conversations or even starting conversations. People want to be noticed and when you notice them they'll typically notice you in kind.
So those are the top three things that come to mind when thinking about my experiences so far with Twitter and what seems to work and what doesn't. Hopefully this helps you out as you explore or improve your author Twitter account. :)