Your Shelf is the home for everything you read.
Tap the + beside "Shelf." Search by title or author. Results come from Google Books, with covers pulled in automatically. Select a result and the details fill themselves in. You can also add a book manually if you can't find it, and upload your own cover image (up to 5MB, cropped to a standard 2:3 book shape).
Every book you add gets a Thought created alongside it automatically. You never have to set that up.
A book can move between any of these at any time. The one exception is a Thought that has been permanently locked. See The Thoughts System.
Results come from Google Books. If a title alone doesn't surface the book you want, add the author to your search (e.g. Piranesi Susanna Clarke). This happens with less famous books, or titles shared by many editions, and adding the author almost always narrows it down. Still nothing? Use add manually and type the details yourself.
You can replace any cover with your own image. Open the book, edit it, and use the cover's upload control. Specs:
An uploaded cover always takes priority over the one pulled from the web.
The book you mark as Reading can appear as your "Currently Reading" on your profile, if you leave that setting on. It's the one small, ambient signal friends see, never the content of your Thoughts.