Launch the App
Go to Library
Slide to My Library or Reading List
Then Tap and hold on the book you would like to delete to select it
A delete (trash) Icon and share option will pop up on the top right section of the screen.
See screenshot below
Click on the Trash Icon to archive the book
The book will be deleted/archived from your Reading list (or library)
You can find archived/deleted books by clicking on the Downward arrow on the top-right section of the library.
See screenshot below: