I wrote an in-depth guide to mashup construction that was published by Wiley as part of their ExtremeTech Series. The book explains everything from the basics of music theory and song selection to more advanced topics such as acapella extraction and mastering. Check out the pdfs for the table of contents, index, and chapter one. Chapters 5 and 13 have also been posted on extremetech.com.
Purchase it at your local independent bookstore, or on Amazon.com