Honestly, the Nikon 5000 you have is actually quite a good camera.
I would start with http://www.strobist.com and go through the lighting tutorials. Much of what you learn in there about applying light artificially will help you "see the light" in the field.
There are a number of good books on photography, but you do not need all the technical aspects anymore. Now, don't get get me wrong, as I'm not saying they're useless, but the percentage of time you'll need those is very small. You see, the gear these days, including that d5000, will do a whole lot for you. What it will not do, and what will set you apart from everyone else, is your composition, subjects, lighting, angle ... all of the artsy stuff. Do you need to know the exact ratio for Rembrandt lighting? Not usually. Do you need to know WHY the stops go f1.4-2-2.8-4-5.6..? Not really.
You WILL need to know the relationship between shutter-speed, ISO/EV and aperture. But you will find this out by playing around and experimenting. Many will say "yes! That shows you need to go read a book!" I say "no, it does not. The book will give you the cold details but the creative warmth and innate FEEL for what it is doing is worth more."
Develop your eye, and the art side. That d5000 won't get in your way for a long long time. And once it does, it has plenty of options and power to take you to the next level. As you start shooting you'll start to wonder, and when you wonder and play you'll seek out that knowledge and learn. By doing it that way, you'll continue to enjoy it instead of thinking "I need to read these 4 books and study all this stuff ahead of time." Talk about a way to kill a passion ...
Have fun, shoot a lot and see what you like. What you love. You may never need the heavy technical aspects. Don't try to do everything at once ... do it all a little. Shoot some birds. Some rocks. Some long-exposures of cars. Join a photography group so you're not trying to play in a vacuum. Invite friends to go along and shoot with you.
You can read all the books you want on how to play the piano, but until you get in there and plink on it, hear it, touch it, FEEL it ... you won't really know how to play. But many play by ear, by feel, and don't need all the super-heavy (and quite often, super tedious) music theory. The same is with the camera.
I suppose the same can be said for backpacking, too. You can read and read and read ... or just get out there and do it.
Once you're hooked, then the studying and GAS sets in ... >:)