The door he is pointing to opens inwards.
There are books right in front of it on the floor. That means the mess on the floor happened after the door was shut.
He said he was knocked out by the thief and the thief couldn't have made that mess after he left, shutting the door behind him.
It is unlikely that the librarian was holding a pile of books that he dropped on the floor after being for example, punched in his head, falling while the thief made his escape, shutting the door behind him,
as the librarian stated that he tried to stop him with all his strength.
Besides... how could he know that he rushed away, and seeing his escape path if he was knocked out. Knocked down...maybe.
"To knock someone out means to cause them to become unconscious or to go to sleep." - Collins Dictionary
My nickname is gandorgandor and the server I play on is NORDICS 1.