If you are using Lightroom, then Scott Kelby's book The Adobe Photoshop Lightroom 3 Book for Digital Photographers comes highly recommended. It's more for learning all the ins and outs of Lightroom (not just printing) but has many tips for printing as well.
That being said, I find much of what I need to learn about printing on the internet at sites such as Lu-La (this one). Also Eric Chan has some good resources: http://people.csail.mit.edu/ericchan/
(and he's an ACR-LR "guru").
I would be highly remiss in not mentioning Jeff Schewe's books in collaboration with Bruce Fraser on Camera Raw and PS CS5 and the book on Sharpening (http://www.adobepress.com/authors/bio.asp?a=7C76D434-9BFD-4454-AB55-850E49FB3A50
). There are also a list of articles at the same URL.
You can also check out my Lightroom Visual Guide available online