If you want a great example of how to establish your main character in a complex setting while driving you straight into the plot, all in 12 pages, read the HG first book, first chapter. It gets you into the world and action faster than books I've seen start in medias re (even decent ones).

YA and youth writing, when it's good, is excellent instructional material for learning essential brevity and active pacing. (The other things on that vein that I'd say is useful to practice with are soeechwriting, screenwriting, and newspaper journalistic writing -- all with some guidance, of course.)