I was going to add books so that I could review them ... but don't see a way to do that.

Link

Click the button that says "Add a Book I've Read." Voila! =)

If you just look up the book you can rate the book and under the more options tab to the right hit the BIR tab.  It won't put books on your BIR tab unless you give them a rating. You don't have to review them but you have to give them a star rating.

And, to carry Mary's input to the next step, you don't need to mark a book as a "Book I've Read" to review it. 

Just go to the listing, click "More Options" and then click "Write Review" - put in your review, give it a star rating (and a title, if you want), Submit Review and you're done. It will be automatically added to your BIR.