In the preface, Butcher declares, "you will learn everything you need in this book." That's overly ambitious but only slightly so; if you're generally familiar with HTML and pretty much any programming language, you'll be able to follow along without difficulty. Butcher is literate (he's working on a Ph.D. in philosophy at Loyola), a veteran technical writer (five books for Packt Publishing) and a bona fide Drupal developer (for Chicago-based Palantir.net). He knows his stuff and explains it well.
If you're a designer working only in Photoshop and you've never written a line of code, you're going to have a tough row to hoe, but even then you can probably keep up if you put in the energy. Butcher assumes little and explains everything, but does so efficiently. If you need to be told three times, you're going to have to read three times.
As the book moves along, it progresses from being heavy on explanation with brief examples to being heavy on exercises with brief explanations. By Chapter 3 you're extracting data from the DOM, manipulating CSS, and creating content carousels. By Chapter 4 you're writing a text editor, a project you'll keep enhancing later in the course. By Chapter 7 you're fetching structured data across the network (AJAX, JASON, AHAH, etc.) By Chapter 8 you're writing Drupal modules and qualifying for your wizard's hat and gown.
The jQuery library is an open-source project with many benefits, including hiding some of the occasionally ugly differences between the ways two browsers behave. It's been adopted as an official part of Drupal, but also has the backing of Nokia and Microsoft and shows up in many other Web applications.
When it's properly used, it can greatly improve Web usability. I've been looking at Drupal 7, which is not yet released, and it's completely transformed the experience of managing content in a Drupal environment through smart application of jQuery-driven effects. When you want to edit something in D7, you just point at it, click the control that magically materializes, and watch as an editable version appears in a layer, courtesy of jQuery, AJAX and standards-compliant HTML.
If you're a developer and you aren't familiar with these tools, you need to be. And I'd recommend Butcher's book as one of the best-crafted technical training manuals I've come across. Dig in.