*Note: With singular uses by whom/by whom, they will not work because it does not end with an m and cannot be easily associated with whom as it is. To avoid amplifying the binary genre, just use the plural them and them all the time. Examples: Who/Who wrote the letter? He wrote the letter. So, who`s right. Who/Who should I choose? Should I vote for him? Therefore, who is right. We all know who/who pulled this trick. This sentence contains two clauses: We all know and who/who pulled this prank. We are interested in the second clause because it contains the who/who. He pulled this prank. So, who`s right. We wondered who/who was the book about. This sentence contains two sentences: We wondered who/who the book is about. Again, we are interested in the second provision because it contains the who/who.
The book was about him.