If you long press a book cover anywhere in the app or if you press the symbol with the three dots (More) on a book page under the book's cover you open the book menu. This book menu allows you to easily handle your book. You can save a book or remove it, download it to your device, mark it as already read or you can finish it by moving it to "My History".
If the book is part of a series you can easily navigate to the other parts of this series and if there are more books by the same author you can jump to these. Of course, you can also share the book with someone from here.
You can also open the book menu from the audio player or the e-book reader. In the audio player, you press the three dots on the upper right and you will find a few other options like changing your skipping intervals or reporting an error in the book. Here you can also change the player to match your device's theme.