How do I delete my account?
In Settings, under Sync across devices, tap Delete account. The account deletion page explains the rest.
My book is not found.
Try the ISBN from the back of the book, set the search language to Any language, or switch the search between Google Books and Open Library. Neither knows every book; you can always add a book by hand.
Why does the search language not change the app language?
On purpose: many people read in a language other than the one they use their phone in. The search language only decides which edition you find, and it is remembered.
A cover or a title is wrong.
Edit the book in your library; your copy is yours and is not sent back to Google Books or Open Library.
Do I need an account?
No. Without one your library stays in this browser. Download it as a CSV in Settings now and then, or sign in to keep it on all your devices.
Where do the prices come from?
Amazon offers no free price service, so the Kindle button opens a search in the Kindle store of your country (set in Settings, or per book). The price shown is the e-book price at Google Play Books or Apple Books in that country (the cheaper one), checked about once a day when you open your wishlist; you are told when it dropped since you last saw it. It can differ from the Kindle price, and Google does not have every book.