Mouses answer is the correct one. If someone else is going to print that book cover then ask them what requirements they need from you to make that happen. If you havent decided yet who is going to print the cover then get in touch with a few printers and get their requirements for it if you were to send them the business.

Normally that would be CMYK TIFF at 300 or 600 dpi. If your printing something like a poster then it may be that you can use 200dpi or less. If your printing a lot of text then you may need to supply a vectored format like postscript or encapsulated postscript or PDF file (which is basically postscript). You may want to ask about colour calibration profiles as well or else all your colours will come out incorrect.

I agree with the posters here who say that it is important to get this right at the start when doing a bitmapped format like an image. There is no dpi on vector format until you rasterize it at the end. Usually you let the printer rasterize a vector file so its never an issue. If you have bitmapped texture fill in the vector format then treat it as though it were a bitmapped format for that fill. I.e. make sure the fill is of the right dpi.

For my full answer see my tut:
https://cartographersguild.com/showthread.php?t=2596