So, when we did our survey, we found three distributions accounted for 65% of the deployed systems. Additionally, when we looked at the other distributions, all but one were based on the other three. So, you could write a book covering Debian, Fedora and SUSE and hit the vast majority of users.

Interestingly, Debian accounted for 44.6% of the Linux systems deployed while SUSE had 11.4% of the systems deployed. Fedora and Red Hat legacy distributions accounted for 9.2%. The later surprised me.
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A final note

We have discovered that Debian, SUSE and Fedora need more documentation support. So, we plan to show our readers how to administer all three distributions. I'm looking forward to the publication date as well as seeing Linux becoming the dominant operating system on the planet.