I think part of the post's popularity is the framing of the issues. In a world of cannot-dos, Jon drafted a list of can-dos. Employers have the "right to hire on qualifications" and the "right to fire on performance" to name just two.
What about the book itself? This is the book I wish someone had handed me when I first started learning about employment law (had it existed). It covers a lot of legal ground, but unlike legal resource materials it's . . . wait for it . . . readable! I actually enjoyed reading it. Jon sprinkles the sometimes necessarily dry legal issues with interesting examples and stories (probably accumulated over his years of blogging). It covers all of the most common employer-issues in an enjoyable ~250 pages.
Also, the book is written from an employer's perspective, not an attorney's perspective. That means the book addresses practical issues and not just legal technicalities. It stops on each issue just long enough to explain the general idea but then moves on without getting bogged down.
Oh, one more thing that differentiates this book from other employment law resource manuals . . . it's only $18! Employment law resources often run in the hundreds of dollars, so the price is right for this one. If you're looking for a high-level overview of employment law issues, The Employer Bill of Rights does the job.
Conflict watch: Jon and I collaborated with other employment law bloggers on Think Before You Click: Strategies for Managing Social Media in the Workplace. I was provided no compensation for this review - not even a review copy of the book - I just really enjoyed it and wanted to spread the word.