Defining Primary Key

Hello

I have this 3 tables, Customer, Transaction and Book. Since a customer can buy the same book more than one time, I've defined Transaction_Nr as primary key of Transaction and put Customer_Nr and Book_Nr as foreign keys. So the layout is more or less like this :

Customer {Customer_Nr, Customer_Name}
Transaction {Transaction_Nr, Date, Book_Nr, Customer_Nr}
Book {Book_Nr, Book_Name}

So now my question is, would it have been better, to define Transaction_Nr, Book_Nr and Customer_Nr as a combined primary key or to leave Transaction_Nr alone ? What would be the difference ?

Thanks !