how to display list response in dialogflow bot - list

Question

I want to display a list of data from firebase realtime database in the dialogflow web bot
eg: user - i need top 3 selling books
Bot - below are details of top selling books
- Anna Karenina by Leo Tolstoy
- The Great Gatsby by F. Scott Fitzgerald
- Hamlet by William Shakespeare
where the database has following data in firebase realtime database
https://i.stack.imgur.com/x4cwV.png
Note: I want to display this bulleted list in default web bot (not google assistant or FB or slack integration)
Have tried below functions, but didn't work
try 1:
return admin.database().ref('books').once("child_added").then((data) => {
agent.add(`${data.val().title} => ${data.val().author}`);
}); // this displays only 1st row of DB
try 2:
return admin.database().ref('books').once("value").then((snapshot) => {
let books_list = snapshot.val();
agent.add(`${holiday_list}`);
}); // this displays the output in json format including the key which is not required
try 3:
var bookRef = firebase.database().ref("books/");
playersRef.orderByChild("title").on("child_added", function(data) {
console.log(`${data.val().title} by ${data.val().author}`);
agent.add(`${data.val().title} by ${data.val().author}`);
});// this prints the required result, but in google cloud console and not in chat-bot
any help is appreciated