You become a competent copywriter by learning from the masters. The principles are universal, and they'll work for any industry.
Next, if you want to write about different topics, it's simple, you make an in-depth research about each and every topic. After you know the ins and outs of whatever it is you're trying to sell, you just take your keyboard and start typing
For great books, I would recommend studying the very classics. Start with Breakthrough Advertising from Schwartz, the best book on copywriting imo.