I picked up What’s Your Pronouns because I was fascinated that the book even existed. I had just had a conversation about why we use plural verbs with the singular they. I found an answer…sort of. I also found a lot more than I imagined. 
 
Dennis Baron has been studying pronouns for 40 years. To me this is the most amazing fact of the book. The first half of the book focus on how pronouns, specifically the use of he, has been used to limit the rights of women. 
 
The second half of the book focus on the use of singular they and it’s long history in the English language. Other non-binary pronouns are mentioned as well. Also, other languages are glanced on briefly. However, mainly this section is why “he or she” is not good and the use of singular they. Ultimately, the answer to my question is we use plural pronouns because they have always been used. However, that is changing. If it changed enough then it will change the “rules.” Also, using singular verbs will stop people complaining that singular they breaks grammar rules (my thoughts not the authors.) 
 
Ultimately, it was an interesting book that feel into the trap of most focused non-fiction books. It repeated itself to make sure it was long enough to be a book. It was still a great read, and one I think more people should pick up.