There is no such thing as "digital photography". Digital is just another medium in what you're really doing, which is, simply, "photography". You need to learn about photography.
There are VERY few general, introductory photography books that I have ever seen that are any good. They tend to be full of gimmicks and, in many cases, outright bad information. The amount of things that get published about photography that are factually incorrect is staggering.
There is only one general photography book that I recommend: "The Camera" by Ansel Adams. Of course, Ansel Adams never picked up a digital camera in his life, and there is certainly nothing about them in the book. That doesn't matter. You need to learn photography.
Then you need to learn the medium of digital, so you can translate what you know and make use of it. Unfortunately, I know of no books on this subject that are good. The field is full of people trying to make a buck off peoples' desire to buy their way into better pictures, or looking for some kind of "system" of getting good pictures, or whatever. It's kind of like personal finance in this respect.
Frankly, you can do pretty well learning the "digital medium" on the web. Sites like dpreview.com, Luminous Landscape, and others are very helpful.
I'm not a professional photographer. My qualifications for saying this: I've been doing photography seriously since I was a teenager; it was my major in college; I've tried it as a profession.
Why am I not a professional? As a business, it sucks. I figured out that I can make more money for less work doing just about anything else.
The average people who make a decent living at it (weddings, portraits, etc) aren't photographers, they're businesspeople. Those businesses aren't about photography, they're about selling. If that is appealing, you can certainly make a living from it, but don't fool yourself that you're turning your hobby into a profession.
The people who do it for a living where it IS about photography work very hard, and don't get paid very much unless they are extraordinary in some way and also fairly lucky. (Think about actors. A few of them make millions; most of them wait tables to make ends meet.)
You say you do nature and scenery. So do I. There is NO market for this stuff any more. The problem is that, in the past couple of years, the market has been flooded with amateurs who are willing to give their work away for next to nothing, or in some cases, actually nothing. In photography, the only difference between "professional" and "amateur" is the money -- an amateur has access to the same equipment, can be just as talented and skilled, and often has MORE time to devote to it simply because he's doing it for fun. So amateur work can be just as good as professional work, but done by people who already have a job and don't need the money. So, they're happy to sell their pictures for a few dollars just for the ego boost.
Or else they submit to "microstock" agencies that totally rip off photographers, giving them a small fraction of a dollar or two per sale, while simultaneously being part of the problem that has devalued the entire industry. I won't touch those places out of principle. They have contributed to the elimination of an entire profession.
Now, as a result of all this, publishers trawl the internet, contacting one photographer after another asking to use their pictures, commercially, for free. When one says no, they move on to the next, and eventually, someone says yes. And, you have an entire industry that has been decimated as its product has been devalued to near zero. The market is completely different than it was even three or four years ago.
If you want to do portraits or weddings or some other kind of "business" shooting, that's still alive. If you want to do photojournalism, great. If you want to do porn, you'll probably make a lot of money. If you can put a lot of money into it and shoot the kind of commercial lifestyle images that amateurs can't shoot because they cost money, you'll do great. But if you want to shoot nature and scenery, keep it as a hobby. That market is dead.
I submit my stuff to a stock library and everything, but when it comes to nature and scenery and travel, it doesn't even come CLOSE to paying expenses.
I got an email not long ago asking for permission to use one of my pictures in a commercial travel magazine as a full-page spread, and that they would not be paying me anything for this privilege. I really felt like replying, "You want me to donate the picture to you? Who do you think you are, Doctors Without Borders?" I didn't, but man, when commercial magazine editors are asking for free use of pictures, things are pretty bad.