Now that looks like a classic secret passage behind a bookshelf. Does it?

The man is about to grab a book. Is there a special mark to recognize it?
Will it activate the opening mechanism?
...

Well not really. In fact, the man is looking for the Secret Gospel of Mark.

He has been told the passage used to be kept secret from the original manuscript.

Unexpected interpretation of the topic "The Secret Passage". Interesting readings can be found here, and here.
The Secret Gospel of Mark is translated here.