The database consists of follwing tables
1. Books - BookId, Title, Price
2. Sales: SalesId, Date, FK_BookId, BookTitle, BookPrice
Set parent child relationship between Books and Sales table on basis of Books:BookId and Sales::FK_BookId fields.
Define Sales table's BookTitle & BookPrice fields as auto-enter calculation field which gets respective fields data from Books table, using relationship. this way when user selects book from dropdown (associated with Sales::FK_BookId field), the data is populated in BookPrice & BookTitle fields.
Take a look at the Invoices starter solutions that come with FileMaker 11 and 12. Both are set up so that when you select an item on the invoice, the current price for that item is looked up from a products table.