Does using a type guard work for you?

function isBook(obj: unknown): obj is Book {
   return (obj as Book)?.author !== undefined
}

You would use it in your code like so:

<GenericSelect
onChange={(value) => {
  if (isBook(value)) {
    console.log(value.author); // Only accessible if `value` is a `Book`
  } else {
    console.log(value.title); // Only accessible if `value` is a `Movie`
  }
}}
values={books}
/>