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I had the same experience, scaled down by a factor of 10 (it was a quick start book).

The level of depth of knowledge and research was at least 0.5x that of your day job? :)

Genuinely curious about how hard the book was to write/research.

Yes, it was actually a lot more in depth than my day job. But I chose to write a deep technical book rather than a shallow "cookbook".

Unfortunately most readers today are more interested in the latter than the former.