After “Search for Books” click (More…) and add “filter” their you can select a field but you should select “Advanced”. Then you will be able to say “Multiselect filter genre’s value contains This Book’s genre”.
Your multiselect Genre, can you post a screen shot of that element editor. You need to be sure that the values are of type genre and also the field of book was of type genre too. Neither need to be text.
No, when you say This Book’s Genre. In that phrase is Genre being saved as a Genre or as a text? The only reason you would be getting an error is if one is a genre and the other is text. They all need to be the same type to work.
Ok so that is it. Since Genre on Book is a list and you are comparing to a list of Multiselect Genre then the way it was worded earlier would not work. Do you want it to show if any of the genre on book are in the Multiselect Genre’s value?