You got me. I often tweet about books. (when I'm not retweeting bad puns, lifve-tweet TV-shows or football-matches or yell about bad Jack the Ripper theories...) I also follow a lot of authors on Twitter but let me tell you a secret:
I probably have never read a single book of more than half of the authors I'm following (I often plan to somewhen but looking at my tbr-pile that won't be happening anytime soon) and I'm also following some authors whose books I have read but found a bit...meh.
Why am I doing this?
Because all those authors are interesting people and tweet cool stuff. They comment on current issues or just make bad puns I can retweet (often both actually). Of course they also talk about their work and announce new books, special offers and such but they aren't doing it all the time. (Just like e.g. the actors and comedians I follow don't just tweet about their upcoming appearances)
In short: of course you can follow me but if I check out who you are and find that your twitter-bio is just praise for your latest novel and your latest tweets are you telling random people to check out your novel chances that I follow you back are...unlikely.
(And if I notice that you keep following and unfollowing me probably in the hope that one day I will follow back I will block you*)
(Yes this post is in reaction to a certain author doing that recently but as they weren't the first to do it I'm not naming anybody as it would be unfair to single one person out)
*that has happened only once so far and that was ages ago but I'm still annoyed