I tutor students in the use of Excel, and I have found that this Bible is worth every penny.
Walkenbach takes you into every nitty gritty detail, so that you can devise a fairly sophisticated lesson plan for those students who want to take their software know-how to a higher level.
His Bible really lives up to its name. I recommend this book for basic to intermediate classes.
Its only flaws? It weighs a ton. You can give yourself a hernia hefting this tome around!
And it is costly. However, I would argue that if you buy this book, you won't have to buy half a dozen other books, because almost everything you need is here.