Saving Images In JPEG?

I read that resaving a photograph after working with it in say Photoshop (JPEG)- can cause you to loose some "quality" each time you open and resave it? Can any one suggest a safe way to work with one's digital images. Would this quality loss be happening each time an image is opened and then saved? Say for instance you made changes to the size for printing or cropped it? Any good books on this subject?