add nix output for the book

flake.nix

@@ -207,6 +207,35 @@
       packages = {
         default = package pkgsHost;
         oci-image = mkOciImage pkgsHost self.packages.${system}.default;
+
+        book =
+        let
+          package = self.packages.${system}.default;
+        in
+        pkgsHost.stdenv.mkDerivation {
+          pname = "${package.pname}-book";
+          version = package.version;
+
+          src = nix-filter {
+            root = ./.;
+            include = [
+              "book.toml"
+              "conduit-example.toml"
+              "README.md"
+              "debian/README.md"
+              "docs"
+            ];
+          };
+
+          nativeBuildInputs = (with pkgsHost; [
+            mdbook
+          ]);
+
+          buildPhase = ''
+            mdbook build
+            mv public $out
+          '';
+        };
       }
       //
       builtins.listToAttrs