I've bought exactly three books recently --
One was an inexpensive paperback, just because I wanted it. It was a quick read by one of my favorite authors... I couldn't resist.
Otherwise, I've decided that books need to fall into the 'non-disposable' category.
That means things like cookbooks and professional books get to come home for good. They're books that I'll use and refer to after I've read them. If the book is just a "for fun" book -- it has to come from the library -- or be borrowed :).
So -- I've added two permanent books to the household recently -- a cookbook, and a book of recent essays about the ethics of war... So, food and dissertation materials can stay :).
What are your rules, if any, about bringing books into your home??