I made a macOS iconset for Calibre, the ebook management software.
The iconset includes icons for retina resolution, up to 512px. The design was inspired by the Contacts.app icon and the cloth cover of library bindings. The font used is Pfeffer Simpelgotisch Semibold.
The simplest way to use the icon is to drag and drop the linked icns file onto the default icon in the “Get Info” window: find Calibre.app in the “Applications” folder; open “Get Info” on it with ⌘I (or find it in the “right-click” context and “File” menus); and drag and drop (or copy and paste) the preview icon in the top left with the new icns file. To revert to the default icon, select the preview icon and “delete.” Relaunch Calibre to effect changes. You’ll need to apply the icon each time the application is updated.
Update: It’s no longer possible to change the Calibre app icon in this way.