I think they NEEDED to sell 250,000 books for the plan to work. I think it gives a clue to how they thought their harebrained scheme would work, anyway.
I assume the book was just a way to get around lottery laws. They didn’t expect the book to sell that well, they expected people would buy it solely to enter the prize draw. The ebook cost £5.
£5 x 250,000 = £1.25 million
After tax, that would just about cover paying off the mortgage and loan (so the property would be free of any charges) and leave them enough to buy a new home of similar value or offer the winner a cash alternative to the house.
I expect the T&Cs specified that the draw would be cancelled if there were less than a certain amount of entries.