I think there are two funnels to this-
1) You want to learn it for yourself (Online Tuts)
2) You have to learn it for your project (Documentation/Reference)
For me, Tutorials are awesome across the board to see someone who's experienced speak to their real-world examples and concepts that you might not have thought of. Gives insight to possible best practices and "gotchas".
On the other side, you might have a particular project where you have a roadmap, specs, etc, and can get right into the Documentation to accomplish it. In most cases, I'm doing both #1 and #2 above.