While Drupal shared many of the same features of other CMS at the time, it's a bit hard to say why it became more popular, but it is probably partly because of what it avoided, namely forking and a reputation poor security that plagued other early CMS. The accessibility of PHP is a bit of double-edge sword; around the time Drupal arrived in 2001, PHP-Nuke had been forked a number of times; in a 2003 article in php|architect, XOOPS's John Cox, a former PHP-Nuke contributor, basically said that it was great how easy it was to start adding things to PHP-Nuke; but that was the problem: just about anybody could do it- and that led to problems cited by him and others to start their own projects. In the open source world people refer to the originator of certain projects as benevolent dictators but while coups are fairly rare, everybody is free to start their own country. In Drupal's case, its originators seem to have avoided the splits that plagued the development of other CMS system, focused early on security, and kept a tighter rein on development.
But besides what they avoided, Drupal offers a lot of functionality and flexibility built-in. Like WordPress, a lot of Drupal's power is found in the modules, both those included with the core release (blogging, comments, polls, etc.) as well as external ones. Beyond that it's quite flexible, works with a number of different databases, there's a choice of templating languages, most parts can be overridden if needed and other features that will appeal to developers. That said, this isn't a book about writing modules, the Drupal site has fairly good documentation on that now; rather it concentrates on showing administrators how to use various modules available for Drupal.
Teach Yourself Drupal goes back and forth between administrating a Drupal site and adding and configuring modules, as well a good overview of how Drupal works internally. Roughly, the book could be broken down into chapters that cover administrating a site, the underlaying structure of Drupal and building your own site.
The book's structure makes following along fairly easy. After the first two chapters on set-up and the basic principals of how Drupal is organized, the book moves back and forth between regular administrator tasks (adding users, setting permissions,etc.) and more "under the hood"-type tasks like using the Content Construction Kit (CCK) to add your own data types to a Drupal site. While Drupal 6 is the current release, the book provides a fair amount of coverage of Drupal 7 which is expected out soon. There are a lot of screen-shots which are handy as a lot of Drupal administration is filling in web forms. One thing some readers will notice is all the screen shots are of Drupal running on the MacOS --which seems to be a bit of trend lately--, more useful is that there is some coverage of installing Drupal on MacOS. Like others in the "24-Hours" series, each chapter ends with a Q/A and short quiz.
If you already know a bit about CMS and Drupal, you'll probably want to skip to chapter 17 (or 16 if you want to build and e-commerce site with UberCart). From Chapter 17 on there's coverage of how to arrange blocks, menus, even a bit on theming.
The book is aimed at people who are comfortable with installing web applications and are looking to set up a Drupal website. Theming Drupal is touched on in the final chapter, but the advice here is fairly basic. If that's your interest, there are several books on that topic alone. If you are new to Drupal, Teach Yourself Drupal gives a fairly comprehensive understanding of how Drupal works and provides plenty of practical advice on getting started.