The book recommended by Ben Wilmore is more to do with the use of digital cameras. It's by Scott who is a good teacher. He is also a Nikon user so that may be of interest to you. If you look at it on the Amazon web site you can preview the contents, and see a sample chapter.
I doubt it'll help a great deal with Photoshop though.
You might also want to subscribe to Photoshop User TV.
http://www.photoshopusertv.com/