@JSWolf: You misunderstand me; but then I suppose I should have clarified better that I considered it different from watermarking. Of course it is trivial to remove, and that's why I call it an ex libris and not a watermark. It's just a way of personalizing a copy of an ebook. As such its function is as a psychological barrier against piracy. Hopefully, would-be pirates will think twice before uploading a book whose second page says something like
"This book belongs to John Smith, who apparently has trusted you enough to lend you a copy. Wouldn't it be ungrateful to redistribute it? Of course it is easy to rip out this page, but can you be sure of removing all kinds of hidden watermarks in this book? How would
you like it if a book traceable back to you appeared on Pirate Bay?
"