I recently finished a shortcut from O'Reilly as well, Using Microformats. Weighing in at only 45 pages, this one is packed with all kinds of good information. As I read it, a number of little things (pardon the pun) sort of popped into place. I picked it up because I was interested in learning enough about microformats to start thinking about them for some personal applications (book reviews, interviews, and genealogical data). While I'm not ready to start designing yet, I certainly feel a lot smarter.
Three sections of the book that I found especially useful were; "Elemental Microformats Catalog", "Microformat Design Patterns", and "Compound MIcroformats Catalog". Having these examples cemented my understanding of the rest of the text. The information in "The Value of Semantic Markup" and "Syling Microformats" should also prove useful as I expand my own use of microformats.
Using Microformats is a good little book on the topic. If you're looking at adding some structured information to your HTML, it will make a great guide as you get started.