Share 'Reviewing Series books'

I'm not quite sure how to tackle reviewing series books, particularly those that are continuous, (ie, Sookie Stackhouse/Gossip Girl). I try to give each book its own analysis and small plot summary, but my reviews seem to get shorter with each book because I don't have as much to say as I read the books. What do you do when you reviews series books? Do you give each book a full review?