This is a good book, and I've wanted to write a review for quite a while now.
To address the "It dosen't support Mac" folks, well that's true -- but realize that the author is really teaching you concepts here, and those concepts will translate to any computer and any software that you're using to do your mashups and remixes. If you want somebody to hold your hand and walk you through everything, then you won't make it very far in music or the music business.
Having said that, I feel this book is excellent, especially chapters 7 thorough 10. You can not only apply this to mashups, but remixing as well, and coverage of subjects like 'unmixing' I've never seen anywhere else.
If you're just now getting into making mashups and remixing, pick up this book. At the current price, it's an excellent tutorial and you'll learn some things here you won't see in other places or books.