For composition, nothing beats going out and getting practical experience. However getting out touristy photo books of your area, or landscape photography in general, and trying to understand what makes paticular shots in them look good is helpful too. Its just like research in any area, it usually helps. look for lead in lines, placement of dominant features, how your eye moves into or round the image etc.
Check out your local library, they may have some books on landscape photography.
Generally GND's are a must if you are getting into lanscape's, especially if you are shooting in the early morning or afternoon/evening when the sky is much brighter than the ground. They are far better than PP, as if they sky is blown out, it is impossible to recover in PP. Very generally it would be extremely difficult/impossible to take good sun set shots, with the forground not in complete blackness without them. HDR could also do it, but I find they never look natural. If you are going into GND's keep in mind that the cheap brands, cokin, hitech etc will give your photos color cast, which will effect the colors in your image, often it will give the image a magenta tone. Filters such as Lee's dont have a color cast, but you will be paying near the cost of a new lens for a set of them.