Writing (from a quick skim check) at least 16 books over a period of 6 years, and earning money from that, is not exactly what I'd call "passive income". And given the topics are mostly (all?) about niches in current technologies they also won't continue to deliver passively forever.
A more appropriate title and angle might be "How I've earned $10k/book and manage to keep writing more while maintaining a day job". (Which is in itself impressive!)
A more appropriate title and angle might be "How I've earned $10k/book and manage to keep writing more while maintaining a day job". (Which is in itself impressive!)