I would imagine with publishing rights, printing costs, shipping costs, demand (or lack thereof) ~ contracts with the larger book stores, which can only be Barnes and Nobles in the States, the loss of sales to the internet...
It probably gets damn costly. It's a good thing that you don't have to hire an illustrator.
I wouldn't imagine your profit margin is huge.
I wonder how Bill Pearl is doing with his books these days? At one point "Keys to the Inner Universe" was doing better than Arnolds Encyclopedia. I think Clarence Bass' "Ripped" series finally tanked. John Little has diversified quite a lot, and Mike Mentzers' old publicist and marketing agent still plugs away at his almost dead website trying to sell any little thing she can. The business is so small, she actually answers the phone herself if you place an order.
Book Sales in general are just at an all-time low due to the internet. People will read *in the book store* but they don't buy.
I would actually be happy to hear that your books are making you wealthy.