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Go to your Profile tab, tap the menu in the top right, and select Import from Goodreads. You'll need to export your library from Goodreads first — go to goodreads.com, navigate to My Books, and use the Import/Export option to download your CSV file. Then bring that file into The Stacks.
When creating a shelf, you can set it to private or public. You can also change a shelf's visibility at any time from the shelf's settings menu (tap the ··· icon). Reviews have the same toggle — you'll see a public/private option when writing or editing a review.
Tap the Book Clubs tab and use the + button to create a new club. You can then invite friends by username and set a current read for the group.
Go to your Profile tab, tap the ··· menu in the top right corner, and select Delete Account. This permanently removes all your data.
On the login screen, tap Forgot password and enter your email address. You'll receive a link to set a new password.