As a former hiring manager, I weighted highly any evidence of an ability to sustain work on projects and bring them to completion (which could be demonstrated on technical and non-technical projects); and evidence of actual interest in technology beyond that it pays the rent (which evidence was more typically a software project, but could be writing or speaking).
Beyond these attributes as proxy measures for job-related abilities, having someone like this around can also help attract and retain other quality engineers — so long as the side hustle doesn't displace the work responsibilities (unless the job is re-cast such that those coincide).
I would expect technical books would be great from a hiring perspective. In addition to the pros you mention (sustained work, etc) if the technology from the book lines up with the job, someone technical can review parts of the book and tell pretty quickly where the person was technically at the time they wrote it. The potential downside for the author being that if they've grown significantly that won't be borne out in the text.
Beyond these attributes as proxy measures for job-related abilities, having someone like this around can also help attract and retain other quality engineers — so long as the side hustle doesn't displace the work responsibilities (unless the job is re-cast such that those coincide).