See full article at The Bookseller.com, http://www.thebookseller.com/in-depth/feature/64194-the-12-books-you-must-stock-.html
(Note that this represents UK sales.)
"Nielsen BookScan’s “evergreen” listing is arguably the most exclusive chart in bookselling history. It is not a rundown of the biggest-selling books, although the titles do shift enormous quantities.
No, it is a list of the books that week after week prove enduringly popular; those that for the past 13 years have managed to remain in Nielsen’s top 5,000, the chart that has been called the Total Consumer Market (TCM) since 2001.
Out of just over 1.8 million different ISBNs that Nielsen BookScan has UK sales data for from the past 13 years, only 12 titles have appeared in every weekly top 5,000 titles chart. That is more than 650 weeks of solid, and often spectacular, sales.
Titles in the evergreen chart would have to shift at least 100 units per week in order to remain in the TCM’s top 5,000. TCM data covers more than 90% of all retail book purchases in the UK
An important caveat is that the data refers to specific ISBNs rather than “the book”. The Very Hungry Caterpillar (9780241003008) refers to the miniature board edition, which has sold close to 600,000 copies to date; sales in all editions of the book top 1.2 million. Titles commonly fall off the list when a new edition is brought out"
Birdsong by Sebastian Faulks (Vintage)
The Very Hungry Caterpillar by Eric Carle (Puffin)
The Hobbit by J R R Tolkien (HarperCollins)
Complete Cookery Course (v 1–3 in 1v) by Delia Smith (BBC Books)
We’re Going on a Bear Hunt by Michael Rosen (Walker)
The Celestine Prophecy by James Redfield (Bantam)89
The Colour of Magic (Discworld) by Terry Pratchett (Corgi)
Long Walk to Freedom by Nelson Mandela (Abacus)
A Brief History of Time by Stephen Hawking (Bantam)
The Road Less Travelled by M Scott Peck (Arrow)
The Light Fantastic (Discworld) by Terry Pratchett (Corgi)
Mort (Discworld) by Terry Pratchett (Corgi)