Put in work once, get paid over a long span of time in a way that is not proportional to the effort you put in at the start. Like any other passive income.
Except he's working every day over 6 years. That doesn't sound very passive
For _multiple_ books. A book he wrote 6 years ago can still earn him some money, without him touching it.
The books are about (versioned) technology. They have a lifespan. It's not like books which which are here to stay for decades