I’ve hired a book author, worked side by side with internet-famous lead devs of two different popular open source projects, and followed the lead dev of another semi-popular open source project that shows up on the HN front page frequently.
The key thing I learned is that they were all very talented at what brought them success in their endeavors, but none of them actually made great developers within a company. In two cases, it became immediately obvious that they wanted to treat their job as a side project (that pays the bills) while they further expanded their book/course/open-source empire.
Being the author of popular educational material doesn’t necessarily make someone a great employee. It can actually present a significant distraction, which unfortunately can make it difficult for them to get jobs.