To purchase an additional book in your subscription, follow these steps for both iOS and Android app versions.
For the iOS app:
Click into the 3-lined menu bar in the upper left-hand corner.
Click "My Books"
Open your Monthbook Subscription by tapping on it.
Click on the month you want to purchase again. (There will be a "Buy again" message under the monthbook.)
Review order details.
Click "Place order"
For the Android app:
Go the Home tab of the Chatbooks app.
Open your Monthbook subscription by tapping on it.
Tap on "Add books".
Additional books will then appear under that button.
These how-to videos will also show you how to purchase additional books from your Subscription as well:
iOS:
How to purchase additional books
For Android:
How to purchase additional books