If you can have the books printed on demand, when the orders come in, it would be worth doing. But in order to get a decent book for a decent price, you will need to order in bulk, usually from a foreign printing company, which adds (very costly) transport costs.
Speculating that perhaps a few customers may want to purchase an additional book is just not cost effective at all. You just end up with a large pile of expensive, unsold inventory.
Modern printing processes have helped to lower the price, but a good quality color print, with a good quality binding still costs more than is worth doing for a limited run. In Jan Still's case, he is currently rewriting one of his older books and it will be on the market in the near future. So it makes more sense for him to focus on producing an updated, extended new book than to republish his older work.
Unless you are writing best sellers, books don't make you rich. Unfortunately
