Changing dpi without changing size

I have tiff. files that are 72dpi. In order to put them into my book for publication they have to be 300dpi. How do I increase the dpi without changing the size of the image. When I go to PhotoShop and change the dpi the picture of course shrinks. Then when I put it into my document and enlarge it to the appropriate size the dpi goes back down.Is there a way to keep the size but put the dpi at a level the printer wants.