Interesting! I saw the book a few weeks ago and wondered who put it together. I assume the letters are public? How did you get the extra material? And do you share any income with Buffet? (That would feel strange, but would also make sense)
The income is not shared with Buffett, but any income I receive is "return on capital" from me taking inventory risk. So I do have to invest some of my $$$ to buy the book inventory, and as it's sold to Amazon I make (great) return on that money. But the profit per book is actually pretty low compared to other published books -- I could be charging much more for the book, but it was part of our "agreement" that I make the total price of the book as cheap as possible. (Which I think is accomplished with Amazon's price of $19-25 for a 700 page textbook-size book.)