I've been spending a fair amount of time working with microdata recently and I've found this Google Structured Data Testing Tool an incredibly useful to have a tool to help me test whether or not my markup has been correctly written.
This post from html5 Doctor is more than four years old, yet it's still one of the best resources on microdata that I’ve found.
Reading through a spec is not always super useful for actually understanding how to use it, but it isn’t a bad thing to have on hand.
Here’s a brief introduction to the concept of microdata from Google. It’s a nice resource with a handful of examples attached. SEO is definitely something that microdata improves so info coming straight from the horse’s mouth is something we should pay attention to.
This is the where the microdata spec is documented. In general, I'd say that microdata can be a bit confusing to start, but this section of schema.org is quite insightful (as compared to the actual documentation). I highly recommend reading through this area before digging to far into microdata.