May I suggest a book. I've used these author's books as my reference since Photoshop 3, and still have my Photoshop 3 copy.
Real World Adobe Photoshop CS3 by David Blatner, Conrad Chavez, and Bruce Fraser

No, they don't have a CS4 book out yet, but I have no doubt they will. In the meantime, it won't have changed essentially from CS3.

From the description on Amazon
"Anyone who relies on Photoshop to meet tough production challenges knows that when a new version arrives, especially one as chockfull of enhancements as Photoshop CS3, there's no time to fool around. You need to get up to speed on it, and you need to do so now. This book will get you there. In this copiously illustrated book, best-selling author David Blatner offers shows you how to sharpen your Photoshop production techniques and provides clear explanations of the concepts that drive them. Readers will learn about managing color, correcting colors, capturing images, getting great scans, and more. They'll also find complete coverage of Photoshop CS3's new features: a streamlined interface, nondestructive Smart Filters, a new Quick Selection and Refine Edge tools, improved Curves, automatic layer alignment and blending, and more. Throughout, the emphasis in this book is on efficiency: the timesaving tips and professional shortcuts that will allow readers to solve their everyday production challenges creatively with Photoshop CS3."

For me, these guys are the best in terms of learning Photoshop in terms of actual working conditions.

Talent is God's gift to you. Using it is your gift to God.