Developer Wil Shipley noticed, too.
In an interview with The Washington Post, Shipley complained about the striking similarity.
"But the thing about iBooks is, it's a book-reader. So, of course they looked around, found the best interface for displaying books (Delicious Library's shelves), and said: yup, this is what we're doing."
He notes that he didn't copyright the idea of showing photo-realistic books on wooden shelves, and that if Apple had called ahead of time they would have revealed a secret on one hand, and admitted that the two apps were similar on the other.
"...they can't write someone a check unless they got some value in return. And if they got value, the lawyers would ask, how much was it? How was it determined?"
Before you call "coincidence," note that many former Delicious Monsters employees are now at Apple. Of course, you can't say that this was malicious. In fact, Shipley's assertion is probably correct: They felt that Delicious Library's implementation was the best and ran with it.
In a way, it's flattering. Something he made has been acknowledged by a huge corporation known for design. Still, it's gotta sting. Shipley again:
"But your [designs] aren't really yours. They have lives of their own. So when your designs do change the world, you have to accept it. You have to say, 'Ok, this was such a good idea, other people took it and ran with it. I win.'"