Biography of Severus Snape, written by Rita Skeeter after the fall of Voldemort.
This would be one of the series of trashy biographies written by Skeeter, including those of Armando Dippett, Dumbledore, and Harry Potter.
This book may not actually exist, but Rowling's comment in the Bloomsbury Live Chat suggests that it does:
Harry would ensure that Snape's heroism was known. Of course, that would not stop Rita Skeeter writing 'Snape: Scoundrel or Saint?'