Just use a getter

src/rust-base/src/lib.rs

@@ -97,6 +97,8 @@ peg::parser! {
     }
 }
 
+// Book is a collection of pages and utilities
+
 pub struct Book {
     pages: HashMap<String, Page>,
     current_page: Option<String>
@@ -437,3 +439,4 @@ THE END
         assert_eq!(book.current_page, Some(String::from("START")));
     }
 }
+

src/rust-base/storybook-godot/src/lib.rs

@@ -9,7 +9,6 @@ godot_init!(init);
 
 #[derive(NativeClass)]
 #[inherit(Reference)]
-#[register_with(Self::register_properties)]
 pub struct Book {
     _book: Option<storybook::Book>,
 }
@@ -46,12 +45,6 @@ pub struct StatChange {
 
 #[methods]
 impl Book {
-    fn register_properties(builder: &ClassBuilder<Book>) {
-        builder
-            .add_property::<String>("body")
-            .with_getter(Self::body_getter)
-            .done();
-    }
     fn new(_owner: &Reference) -> Self {
         Book {
             _book: None,
@@ -75,13 +68,14 @@ impl Book {
             None => (),
         }
     }
-
-    fn body_getter(&self, _owner: TRef<Reference>) -> String {
+    #[export]
+    fn get_body(&self, _owner: &Reference) -> String {
         match &self._book{
             Some(b) => b.get_current().body.clone(),
             None => String::new(),
         }
     }
+    
     #[export]
     fn get_footer(&self, _owner: &Reference) -> Footer {
         match &self._book {