Baffled book bugs trying to get into one of their regular haunts were met with a boarded up door this week.
Those hoping to get into Waterstones from Castle Meadow were being asked to walk down to the main entrance in Castle Street instead.
One of the glass panels on the automatic double doors has been boarded up this week.
But there is mystery surrounding how the door was smashed.
A staff member inside the Norwich store - who did not wish to be named - said: "It would have been damaged on Tuesday but we do not know how it happened."
The door remains boarded up but was opening again by 5pm on Thursday.
A workman could be seen clearing up the debris next to the window inside the shop on Thursday morning.
He said he had been outsourced by Waterstones to carry out the work and he knew nothing about the incident itself.
Meanwhile, a sign on the window apologised to customers for any inconvenience caused and instructed them to use the main entrance in Castle Street instead.
Shopper Marcus Moore, 36, who lives in Sprowston, is worried it may be vandalism given a spate in the city street.
He spotted the boarded-up window while passing by on the bus around 10am on Wednesday.
He said: "That particular area has also had the HSBC bank damaged a couple of months ago. I am worried that it may have been done deliberately."
An unnamed staff member at H&T Pawnbrokers - which is located next to Waterstones in Castle Meadow - said: "I noticed it was damaged on Wednesday but it looks like nothing else has happened.
"Fortunately everything is fine here."
Waterstones has been contacted for comment.
A spokeswoman for Norfolk Police said she was not aware of any reports to them regarding Waterstones.
Norwich Waterstones is a two-floor shop which is the self-proclaimed "largest bookshop in Norwich".
The British company was founded by Tim Waterstone and began in 1982.
It has since grown to more than 280 bookshops with more than 3,000 booksellers.
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