Take your books to a supermarket produce section where there is a scale for measuring fruits and vegetables. This will likely be more reliable and accurate than a bathroom scale or any scale designed to weigh human beings.
Place your book on the produce scale and look at the readout to find out how much the book weighs.
Lift your other books one at a time. You can estimate the book's weight in relation to the first book you measured, or you can place each book individually on the same scale if you want an exact figure.
Add each of the book weights together to calculate the total weight for the entire book set.