Every PDF reader treats the file as a stack of images to scroll through. A book is not that, and reading a long one on a screen that scrolls feels like work in a way that reading the paper version does not.
Flipbook renders a PDF as a bound two-page spread — cover board, centre spine, the stacked edges of the pages you have not got to yet. The page turn is a Core Animation transform tracked one-to-one against the trackpad, so the page follows your finger and you can change your mind halfway through the turn. It is not a cross-fade with a curl drawn on top.
Around the reader sits a library, an annotation layer, reading stats, nine themes, and an AI companion that answers from the actual text of the book rather than from what it happens to know about the title.
Native SwiftUI and AppKit, universal build, macOS 26 and up. No backend, no account, no telemetry — you bring your own key for the AI parts and nothing leaves the machine otherwise.