Couch grass and oxalis is it exactly! Remain strong and keep giving away those books - once they take over they're almost impossible to root out, and you find yourself just assuming that anywhere a book can be put, there a book should be - possibly several, balanced precariously on top of each other. (I met someone once whose office, originally a reasonably spacious room, had been turned into a literal maze by thigh-high walls of books. I'm not sure what happened if he wanted one low down in the wall - I should have thought there was a real risk of toppling them all over like dominoes. In any case, I have long considered him a Dreadful Warning.)