545 authors, to be precise.

From the BBC:

More than 500 leading authors have signed a letter to Prime Minister Gordon Brown urging him to confront childhood illiteracy.

Still haven't found a complete list, but some of the authors mentioned in the article are: Nick Hornby, Joanna Trollope, Kate Mosse, Jackie Collins, Poet Laureate Andrew Motion, Ian Rankin and Sophie Kinsella.