From here, you can go anywhere you please! WordPress All-in-One For Dummies is designed so that you can read one book, or all books, between the front and back cover, depending on what topics interest you.
Book I is a great place to get a good introduction into the world of WordPress, if you've never used it before and would like to find out more. Book II is also extremely helpful in giving you insight into the programming techniques and terminology involved with running a WordPress Web site — and that information is extremely helpful when you move forward to the other minibooks in WordPress All-in-One For Dummies.
Above all else, have fun with the information contained within these pages! Read the books on topics you think you already know about — you might just come across something new! Then dig into the books that contain topics that you really want to discover more about.