I keep coming back to these NavPress Bible study books. They're solid, basic and thorough. Not for any special-interest target group, but useful for any Christian believer. Questions are basic enough for a beginner, with others challenging enough for more mature Bible studiers.
Our small group liked it a lot. It begins with background on John's reasons for writing the letters, including the historical setting. That started us out with a good idea of the purpose of the letters. Throughout that study, we were given explanations of various terms and phrases of which we would otherwise have been ignorant. That increased our understanding a great deal. Highly recommended.