Never judge a book by its cover is one of those old sayings that get dusted down and applied to any situation where appearances aren't supposed to count. Except that when it comes to books, we really do judge them by their covers.

After years of being bombarded with carefully-crafted graphic design, any savvy book buyer can usually tell at a glance the type, tone and maybe even the ending of a book.

If the author's name is written in shiny gold, it's probably a thriller. If the cover is dayglo pink, you can be sure you're in for a bit of sex and shopping and if the writing is in discreet type with a moody classy image, then you're in literary fiction territory.

Book jackets are either author-led or title-led. If the author is well known, expect to see their name bigger than the book's title. Patricia Cornwall, Maeve Binchy and Ian Rankin are just a few authors who are bought on the basis of their name and fame. Publishers know there are readers awaiting the new Bill Bryson or Stephen King.

A book by or about a celebrity is sold exclusively on the basis of the person's fame, so the actual title of the book is often entirely lost. A first-time novelist - or one selling into a new market - has a trickier time, having to sell on the basis of the title and evocative graphics.

Sex sells, which is why even literary books sometimes feature provocative covers. As Allen Lane, the founder of Penguin Books, said: if you want big sales, you need "bosoms and bottoms".