Books and courses can prepare you with the fundamental do's and don'ts. But regular testing in your market is the only way to keep in touch with how your customers will react to what you communicate. Regular testing is a good discipline and means that you are always ready for that moment of truth when your company needs results fast, throws a large budget at you and expects you to bring in the bacon. Ignore other people's response rates and focus on your own profitability. Resist the temptation to seek a high response just for the sake of it.
Absolute percentages are meaningless.
It depends on the profit margins of each product, the likelihood of repeat sales, and your related marketing costs in qualifying and following up each inquiry. Percentages are very important when you are comparing your own trends. But setting a goal for a response rate in isolation to profit means your motivation and results will be off the target. If markets and technologies never changed, and all people behaved the same, you could probably write a set of rules and software to do your direct marketing. Luckily that's unlikely to happen anytime soon in this market.