It's a good idea LilyTigermoon, but the problem is it has to be manually done and there are only a few people who can do it. There are a few places already to see what the current book is, and these are all available from the main Library
board, so you don't even have to do the extra clicking to come in here :)
Firstly, there's the Announcements
thread, which we try to keep as up to date as possible. It'll tell you what the future books are too, which means you can start reading them before the month.
Then there's the OBC Thread
which BB already linked you to. The title of that normally changes each month, as well on the most recent page you'll find the blurbs etc of the upcoming books.
Finally, the current book is always in the Library, not in this board. It only gets moved in here when its month is over so that we can easily find the threads if someone wants to restart conversation. Pretty much always the current OBC thread will be on the first page of the Library, as it's a current topic (and if it ever falls off I'll go bump it back up). This is what the Library board looks like currently, I've drawn an arrow to the current thread (click in the image to see it bigger)
Hope that helps :)