I heard about this unusual book recently on BBC Radio 4’s Beyond Belief programme where the author Frank Cottrell-Boyce chose it as his favourite children’s book with a religious message, describing it as ‘an advent calendar within a book’.

Its unique structure takes us day by day through December as a young Norwegian boy opens a door of a ‘magic advent calendar’ he discovers amongst the toy and chocolate filled offerings in his local bookshop.

Each door contains a picture and a piece of paper containing the latest part of a mysterious story in which a young girl called Elisabet follows a sheep out of the same bookshop, meeting angels and various historical figures as she travels across continents and back through time all the way to the birth of Christ.

Who has placed this strange calendar in the shop, and what happened to the real Elisabet who disappeared from the town many years before? Gaarder keeps you guessing to the end. 

For Frank Cottrell-Boyce the Christmas Mystery is a children’s classic.

‘It contains the two things I love about the best in children’s fiction – intellectual boldness and narrative inventiveness.’

Having written a Christmas book (Millions) with its own cast of religious figures, which does Frank think are better, saints or angels?

‘Saints of course! They’ve had to struggle a bit. They’re more rough and tumble and such an odd bunch.’

Like a real advent calendar it is clearly written with the intention of sharing over 24 days, as Frank’s family do each year.

‘My wife reads a chapter of it a night to our children all the way through advent.’

Read in these short chunks it building to a brilliant final revelation. So does Frank think the book also work as a mystery?  

‘Yes completely.  What’s brilliant about it is that the solution to the mystery turns out to be Mystery itself.’

Intrigued? The Christmas Mystery by Jostein Gaarder is published by Orion. Illustrations shown here are taken from the 1996 edition by Rosemary Wells.