JPGs are a final product. Once you have one, from whatever way it was created: in-camera, or a "save as jpg" file, you have final product. Making any changes to a jpg and saving the result as a jpg will cause for a new loss of data (even if saved at the highest quality).
I have always compared a jpg from a camera to a polaroid print: you get a final 'print' processed by the camera processor. It can be quite good, but it is a final result.
Upsizing a jpg is similar to copying a small print and printing an enlargement. All you are doing is increasing the size of whatever made the small print: silver grains, inkjet dots, or whatever. You cannot retrieve any details that were removed from the original capture.
When designing books I am most frequently provided with jpg images, most require some sort of manipulation: sizing, cropping, exposure, etc. I do the best I can, and always save the results as either a tif or psd file. There really is no other way.