This book by Jon Duckett is a beautiful reference book. It's very easy to read, easy to flip through to find what you're looking for, and shows clear examples.
The book is definitely more reference than a tutorial, so I would read through part of it then go online and do some hands-on learning with tutorials from Tuts Plus. I think this was an enjoyable and effective way to learn, and I think I'll even reference my silly little HTML file in the future since it shows examples of a few different things. As I built the file, I also added comments explaining what everything was.
This week my learning priority is definitely fundamental Ruby, so I'm not going to worry too much about HTML & CSS. I'll likely use it as a break from Ruby, but keep it high-level. Next weekend I can focus on it a little more and keep working through the Tuts Plus tutorials.