Добавьте файлы проекта.

BookReader/AppShell.xaml.cs

@@ -0,0 +1,15 @@
+using BookReader.Views;
+
+namespace BookReader;
+
+public partial class AppShell : Shell
+{
+    public AppShell()
+    {
+        InitializeComponent();
+
+        Routing.RegisterRoute("reader", typeof(ReaderPage));
+        Routing.RegisterRoute("settings", typeof(SettingsPage));
+        Routing.RegisterRoute("calibre", typeof(CalibreLibraryPage));
+    }
+}