At one point in my career my employer repositioned his web application development company to become a search engine optimization (SEO) company. This repositioning meant that for a very brief period I actually worked in the world of SEO. I didn't much care for it (hence the 'very brief period' working in SEO). For all the science of keyword research and result monitoring, there seemed to be an equal part of voodoo or tea leaf reading. And, lets face it, SEO wasn't as glamourous as finding an elegant technical solution or designing something beautiful for a client website.
Still, every so often SEO of a site is a client requirement. Even if you ultimately outsource the work of optimization to a third party, if you are a web site developer you need to be knowledgeable about SEO and be prepared to build your site in such a way as to ease the work of optimization. But, with so much information available on the web on the topic of SEO, even approaching the topic somewhat overwhelming.
Luckily there are books like Drupal 6 Search Engine Optimization by Ben Finklea to help you get oriented. Not only is this book filled with valuable information about SEO, keyword research and optimization practices, it does a good job of highlighting all the Drupal specific features, tools and modules available to you. It also includes plenty of step by step instructions to help implement your SEO strategy using the tools available in Drupal.
Score based on my own arbitrary rating system:
For any Drupal Developer or Drupal Site Administrator: 4.5/ 5