-- Arthur Schopenhauer, "On Authorship"
https://en.wikisource.org/wiki/On_Authorship
For professionals in technical fields where skills-assessment itself is expensive and uncertain, a book serves at a minimum as a signifier of one's own skill and ability. In exceptional circumstances, a book can lead to the author becoming part of both the skill-generating and certifying mechanism. Writing "the book" on a topic, and the teaching, lecturing, training, or technical leadership roles availed ... may ... prove worthwhile. Others then display their copies or familiarity of the book as evidence of their own ability. (Knuth, Gang of Four, Kernighan & Ritchie, etc.)
It's critical to realise that this signalling capability, as with attention, is a fundamentally rivalrous and finite space (perhaps not fully zero-sum), such that there can only be one truly leading authority or reference at a time. Though if like Martin Kleppmann you hit that spot, it can prove rewarding.