Think about what valid, useful reasons people have to return to the app. E.g in a game, they might want to see the progress in their crops. In a book collecting

Think about what valid, useful reasons people have to return to the app. E.g in a game, they might want to see the progress in their crops. In a book collecting app, it might be to find out what new books appeal to them, or to enter their review when they've finished reading a book, or when a new book by an author they've read is available, or when their friend has reviewed or added a book, etc.. Figure out how the app can trigger or remind them of these moments. Notifications are the obvious one, but you want to be careful not to train the user to tune out your app if you send too many notifs or notifs they are uninterested in. The best triggers are ones the user has asked for themselves. For example, when they get a new book you can prompt them to set a goal to finish reading it in a week or two, or whatever is comfortable, and then ask them if they want you to check up on them (notification) when that time is up. If they set the goal, they will probably welcome it. Additionally, you want to try and build habits that help the user improve his or her life. So if you build a habit of recording their daily reading log, and reward the user for reading every day without missing one, then they are happy because they are reading more books and they are also connecting their new reading habit with the action of opening and using your app.