If I understand you correctly, what you are after is a belongsTo - hasMany relationship or one to many. It's not polymorphic.
If you read it out: a BOOK belongs to a CATEGORY
A CATEGORY has many BOOKS
So in your book table, you'd have a category_id field.
http://laravel.com/docs/5.1/eloquent-relationships#one-to-many
Just make sure this is definately the right approach and you won't want a book to have more than one category. It would be a pain down the line if you had to change it.
Hope it helps.
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