Is it my imagination, or is there a point where tweeting about your book tips over from promotion to spam?
It is not your imagination.
The trick is to not tweet about your books more than once every ten tweets.
(and yes, a score sheet helps)
The other trick is to let OTHER people talk about your book.
And when other people talk about your book, you don't retweet it. You REPLY to the person who tweeted, thanking them. And if you're super clever, you put a dot in front of their Twitter name:
so that your reply to them is seen by all your followers. Subtle promotion is a better strategy for the long haul. And Twitter is long haul promotion.
Those other nine tweets: that's when YOU talk about other people's books.
Some other Twitter promotion nuances:
(1) Don't sync your Facebook and Twitter feeds. Twitter and Facebook are different audiences, different strategies.
If you want to post a link to your Facebook page on Twitter you write: There's news about the Runner audio edition on my Facebook page (link)
What doesn't work "I reviewed a book: (link)"
(2) Don't treat us to a running commentary on number of followers. Don't run contests to build the number of followers. Don't talk about this stuff at ALL. No one following you cares how many other people are. They ONLY care that you say something of interest to them. This is the single biggest mistake people make on Twitter.
(3) Be super super careful what you live tweet. The reason for that is live tweeting by definition is a LOT of tweets and if you fill up my tweet stream with stuff I'm repeatedly not intersted in, I'll unfollow you (and other people will too)
(4) Cat pictures and local weather pictures are the things that get people's attention for the first time. Use that info to your advantage.