What are the 4 things that you have to do in order to be considered a writer? This is my list for myself, and so far it's worked out pretty well.
Speakers are people who speak, painters are people who paint, and writers are people who write -- not people who say that they will write one of these days.
Read in your genre
If your subject is social mores in the middle ages, then you have to read as much as you can on that subject. One reason is to become and maintain a position as an expert.
Another is to see what others are writing about in that field, in case there are developments that impinge on your own work.
Yet another is to see if there are new techniques in writing on that subject -- and perhaps to invent one of there does not seem to be. To take a case in point, my reading of the situation is that Malcolm Gladwell invented, or at least popularised, a particluar approach to writing non-fiction. It includes detailed descriptions of the people involved, anecdotes, and a conversational tone. The problem is that there are now so many imitators it has become, in my opinion, somewhat boringly predictable. It would be a better idea, again IMHO, if would-be imitators would come up with a new idea.
Similar considerations apply in fiction writing too. If you write YA fiction, you have to keep abreast of what is being demanded now, and what is passé.
Read outside your genre
There is always the danger, especially in non-fiction writing, of ending up inside what is effectively an echo chamber, in which everyone is doing what everyone else is doing (cf my comments on Malcolm Gladwell, above). It's good to see what's going on outside your usual boundaries.
This is usually thought of as mainly the province of cition writers, who always seem to be noting down snippets of conversation and descriptions of people for possible future reference. However, I think non-fiction writers can benefit from this as well.
For example, I always carry a camera around with me in case I spot an unusual sign or an interesting pattern. Most of the time I have no idea whether or not I can make use of them, but carrying a camera forces me to look at the world in a different sort of way. Sometimes that causes me to spot things and connections that I know I can put to good use in my writing.