Best-selling author John Grisham says that his new novella, The Tumour, is the most important book he has ever written because he claims it raises awareness of a new cancer treatment.

The 49-page novella, which is being offered free of charge, is about a man afflicted with brain cancer and Grisham has written it to raise awareness of a new treatment called focused ultrasound.

The lawyer turned author, who has written a series of best sellers including The Firm and The Pelican Brief, is on the board of the Focused Ultrasound Foundation, which believes that the treatment is the best hope for future cancer patients.

Writing on the Foundation's website, 61-year old Grisham says that he knows little about medicine and medical research but when he learned about Focused Ultrasound and its potential to change lives, he knew it was "a story worth telling".

"This is the most important book I have ever written," he says. "I have found no other cause that can potentially save so many lives. One day, in the not-too-distant future, you or someone you love will be diagnosed with a tumor. After the shock, you will think of focused ultrasound. Let’s hope it’s available."

The book can be ordered as an e-book or hard copy on the Focused Ultrasound Foundation website and there is also a 'donate' facility.

The Tumour is the fruit of Grisham's eight-year friendship with Neal Kassell, the neurosurgeon behind the development of Focused Ultrasound.

The book's introduction claims that there is sufficient research to date to be "very optimistic . . . But progress is too slow. There are barriers from regulators, insurance companies, even many in the medical field.”