It's a novel. The structure and style of My Happy Marriage indicate that it originated as a novel, not a manga. The depth of character development and the complexity of the plot are typical of a novel format.
The popularity of the 'my happy marriage free novel' can be hard to gauge without more data. However, if it has been translated into multiple languages or has a large number of downloads on free e - book platforms, that could be an indication of its popularity. Also, if it has a strong following on social media platforms where readers discuss books, it might be quite popular.
It's about a marriage that is happy. It might detail the relationship between the two main characters, how they met, and how they overcame difficulties together in their married life.