That's $6 profit... He got $12000 for 4000 books, which is 50% of the profit, his cut. For a physical book, with distribution costs, and the likes of amazon are probably getting about a 20-30% cut.. so that's really more like $9 profit off a $15 book.. 3 for the author, 3 for the publisher and 2-3 for Amazon. bringing the cost of the book's printing to $6 or so... not bad.

Also, the "best seller" was only for the one week, and 4000 books, of which most were probably during that one week isn't so bad.. the tail probably sucked though.

Now, self-publishing as an e-Book on amazon, would have given him potentially a bit more.. say Amazon takes $4, that gives him $11, and if only 1/2 bought it (available in digital only), would gross him $22k, even with a price drop to say $10 for digital (amazon getting $3) would be $14k ... That's some food for thought. If you can write a book every two months, and sell 2K copies for a $10 digital version.. that's not a bad living.

Bonus, if you become a little well known, people will order more of your back catalog.