The reason they call it a pet peeve...

...is that it's petty. Here's mine:

Lately, a lot of people have been coming up to the desk confusing their prepositions. Just in case you don't remember your elementary school grammar:

A preposition is a word which shows relationships among other words in the sentence.

So, when someone comes up to the desk asking for a book on John Locke, I think Locke, John. Biography. as in subject headings. Then, if I ask for more info, or the patron looks confused when confronted with the biographies of Locke, I ask if they are looking for something specific on him, like a critique of his thoughts on education, or moral philosophy. And they say: "Yeah, my professor said we had to read the book he wrote [emphasis mine] on education." And I say "Oh, you want a book by John Locke".

Yeah, petty...maybe. But isn't there a bit of a difference between the clothes on Coco Chanel and the clothes by her?