Karl Seguin has written a short (thirty-two pages) guide to MongoDB.
It won’t make you a hairy-chested terror at big data conferences but it will get you started with MongoDB.
I would bookmark http://mongly.com/, also by Karl, to consult along with the Little MongoDB book.
Finally, as you learn MongoDB, contribute to these and other resources with examples, tutorials, data sets.
Particularly tutorials on analysis of data sets. It is one thing to know schema X works in general with data sets of type Y. It is quite another to understand why.