but I add some weight to it by wrapping a little bubble wrap around book, shouldn't I take it to the post office and have them weigh it before I purchase the stamps?

When you select the weight, there is a choice if it is the weight of the book or the wrapped book.  So it adds wiggle room.   Having a scale does help as the weight is not always correct.  But it usually is too much rather than too little.

Thank You Emily.

When I weight a book, I round it up a couple of ounces...my wiggle room.

"When I weight a book, I round it up a couple of ounces...my wiggle room."

Unless you're sending it First Class Mail, or adding those ounces causes it to cross a pound boundary (ie: weighs 14 ounces, you add 3 ounces, so now it's 1 lb. 1 oz.) it won't make much difference.  First Class mail is priced by the ounce, but Media Mail is priced in full pound increments.