This is the perfect book for someone who has read other MDX books but still isn't clear on the difference between a tuple and a set. It isn't as detailed as Spofford's "Inside MDX", but spends most of its time giving detailed descriptions (with plenty of graphics) of the basics of MDX. It solved a host of MDX problems for me where a basic misunderstanding was causing an otherwise-correct statement to fail.
This is not a detailed reference, and you'll also need a more comprehensive book such as Spofford's to go past the basics. But for a "first book" on MDX, this is the right choice.