The product is called "Cthulhu and the River of Doubt". It has been available as a download or print addition from Lulu for quite a while, but as I said it's now available for the Kindle.
So what is it you ask! Here's the book blurb :
This book is designed for the Role Player with a historic Horror bent, specifically for “Call of Cthulhu” but can be read by anyone. The bulk of the book is made up of the entire text of “Through the Brazilian Wilderness” written by President Theodore Roosevelt. His writing describes a journey taken by himself and acquaintances in 1913, through the steamy jungles of Brazil traveling down the uncharted River of Doubt. This was no pleasure cruise. This journey was an exploring adventure of toil, fear, danger and murder. The jungle that surrounded them for months contained dangerous animals and threatening indigenous people, the water even held man eating fish. Footnotes appear throughout detailing elements and suggestions for turning this epic journey into a Role Playing campaign. An appendix at the end suggests some character profiles for the main protagonists. The Roosevelt-Rondon expedition is the perfect historical source for a role playing campaign.
If you're a regular listener to the podcast you'll know that the Roosevelt-Rondon tale is being serialised here in audio format. So what you'll be getting is the text of the original book with annotations by myself added in.