I have something like 125 credit hours in photography. I've seen a LOT of photo instruction books and IMHO, photo books are generally useless unless you want to cover the absolute basics. Go to the library. It is all on the web too. Ken Rockwell is a hoot on camers and techniques-- read his stuff: http://www.kenrockwell.com/index.htm
You should understand the relationship to shutter speed and lens aperature, focusing techniques(read up on hyperfocal distance), macro photo techniques, and using flash (thou shalt know thy Inverse Square Law). With digital stuff, understand how "noise" is created and how to use it or avoid it. Understand dynamic range and how your camera can be tweaked to get what you want from it.
1) Go to the library, pull a dozen books off the shelf on the subjet matter you want to photograph and look at them all. Then go get some more..... rinse and repeat :) Every time I took on a different subject matter, I found experts in that field who were really good at making photographs, because they knew what was important-- say, the differences in petal color patterns between two kinds of orchids, or the behavior of the Western Ice Worm. Someone trained as a photographer *might* have a leg up on the creative side--- composition and such, but those experts know what works and they have seen forty zillion photos on their subject.
2) Old National Geographics will teach you a lot. Those folk are really good at making and *editing* images.
4) Don't just thumb through images: study them, take them apart, analyze them and know why some work and some don't. Pay attention to camera angle, angle of view, color, exposure, natural and added lighting. If you can't figure out how the photos were made, then you can ask some questions.
5) Take LOTS of photos, varying angle, exposue, focus and focal length. Analyze them just like you did with the photos others made. Be brutally honest with yourself. Learn from seeing and doing.
Basic nature photo things to know? Use a tripod and remote release. Study the light--- that means getting up VERY early. Know how to use a flash for fill light. Know how to use a reflector for the same. Know how to do good macro work. Know how filters work-- polarizers are big.
Have fun too!