The easiest way to add chapters to already-generated books is to use the chapter sidebar in the Bookle editor.

Follow the steps below:

Step 1: Click the (...) menu on the chapter you want to add another one close to, and decide if you want it above or below that chapter.

OR, Add a chapter to the end of your book using the (+) button at the bottom of the list in your chapter sidebar.

Step 2: Click Save.