Is there any authenticity to the book 'The Deathbed Sutra of the Buddha: Or Siddhartha's Regrets'? I found this book in a local bookstore and went online to research it. I could not find much reference to it.  The author, George C. Adams Jr., purports to have been given a lost sutra of the Buddha, given on his deathbed, to Ananda. It looks like a work of complete fiction to me but was wondering if anyone has more information on it.
Deathbed Sutra of the Buddha
From the customers' reviews of the book, I get the impression that it is a fictional work to provoke a reassessment of one's thinking in light of controversies.
A few reviews from customers:

This book a very clever merging of philosophy and fiction, which examines and challenges some fundamental Buddhist beliefs in a manner that manages to be both deeply insightful but also delightfully entertaining. It’s a short work, but packed full of dense analysis of Buddhist thought, using the clever device of a fictional conversation between Buddha and his attendant, Ananda. Readers are likely to love it or hate it, but in either case it’s definitely a great read.
While it's not a &quot;DaVinci Code for Buddhists&quot; as stated on the back cover, it does offer a fast moving narrative conversation that moves along through the final night of the Buddha's life. It left me wondering what will happen next, and what will the Buddha say next? Thought-provoking and fun at the same time.

MVFWIW is if you are new to Buddhism then it is advisable to learn about the authentic teachings first and when you have cultivated wisdom and faith then to challenge that faith and wisdom with controversies.
As it is Theravada, Mahayana and Vajrayana have provided enough divergent materials for your wisdom and faith to work on.