Written books resolves the JSON text when being opened by the player. If an op opens the book, selector components resolve correctly. However when a regular player opens the book, it causes an exception which causes the literal JSON to be displayed in the book instead.

Steps to reproduce:

Execute the following command

/give @a minecraft:written_book{title:"Example",author:"someone",pages:["{\"selector\":\"@s\"}"]}

If a op opened the book, it would display their name as the contents. However if a regular player opens it they will see {"selector":"@s"} instead.