Pantone Swatch Guides - but currently their Process to Solid Color guide is out of stock - a good way to see printed CMYK values as well as pantone values.
I have found
this book to be an amazing resource.
You don't want a digital reference for CMYK because you wil still run the risk of inaccurate color matching between your monitor and press, which may only become apparent in the proofing stage, causing you unnecessary delays.