The new edition of this Missing Manual covers the new version of Dreamweaver, which was released by Adobe this spring, Dreamweaver CS5.5. If you have Dreamweaver CS5, then you don't need the new book.
If you're thinking about upgrading to Dreamweaver CS5.5, you can find out what's new in this version by checking out this section from the CS5.5 Missing Manual and this post from Adobe Developer Connection.
I hope this helps. Have a great day!