diff --git a/src/preprocessor.rs b/src/preprocessor.rs
index cf0f18c..c65245b 100644
--- a/src/preprocessor.rs
+++ b/src/preprocessor.rs
@@ -61,7 +61,7 @@ impl Preprocessor for CheckCodePreprocessor {
         "check-code"
     }
 
-    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
+    fn run(&self, ctx: &PreprocessorContext, mut book: Book) -> Result<Book> {
         // Check if book.toml is approved
         let book_toml_path = ctx.root.join("book.toml");
 
@@ -115,9 +115,11 @@ impl Preprocessor for CheckCodePreprocessor {
         let mut failed_files = HashSet::new();
         let mut stats: HashMap<String, usize> = HashMap::new();
 
-        // Process each chapter in the book
-        for section in &book.sections {
-            if let BookItem::Chapter(chapter) = section {
+        // Process all chapters recursively (including nested ones)
+        // Using Book::for_each_mut() is the standard pattern in mdBook preprocessors
+        // and automatically handles traversal of chapter.sub_items at all depths
+        book.for_each_mut(|item| {
+            if let BookItem::Chapter(chapter) = item {
                 if let Some(chapter_path) = &chapter.path {
                     let full_path = src_dir.join(chapter_path);
 
@@ -136,7 +138,7 @@ impl Preprocessor for CheckCodePreprocessor {
                     }
                 }
             }
-        }
+        });
 
         if !failed_files.is_empty() {
             let now = Local::now();
diff --git a/tests/fixtures/src/SUMMARY.md b/tests/fixtures/src/SUMMARY.md
index 1ba61d5..0e7072c 100644
--- a/tests/fixtures/src/SUMMARY.md
+++ b/tests/fixtures/src/SUMMARY.md
@@ -1,10 +1,14 @@
 # Summary
 
 - [Introduction](intro.md)
-- [Parasol C Basic Example](basic.md)
-- [Parasol C Propagate Example](propagate.md)
-- [Plain C Example](c_example.md)
-- [TypeScript Example](typescript_example.md)
+- [Parasol C Examples](parasol_examples/README.md)
+  - [Basic Example](parasol_examples/basic.md)
+  - [Propagate Example](parasol_examples/propagate.md)
+- [Other Languages](other_langs/README.md)
+  - [C Examples](other_langs/c_examples/README.md)
+    - [Plain C Example](other_langs/c_examples/c_example.md)
+  - [TypeScript Examples](other_langs/ts_examples/README.md)
+    - [TypeScript Example](other_langs/ts_examples/typescript_example.md)
 - [Solidity Example](solidity_example.md)
 <!-- Uncomment to test error output: -->
 <!-- - [Test Error](test_error.md) -->
diff --git a/tests/fixtures/src/other_langs/README.md b/tests/fixtures/src/other_langs/README.md
new file mode 100644
index 0000000..ece8f96
--- /dev/null
+++ b/tests/fixtures/src/other_langs/README.md
@@ -0,0 +1,16 @@
+# Other Languages
+
+This section demonstrates code validation for non-Parasol languages at depth level 1.
+
+Code validation works with multiple language configurations. Here is a simple C example:
+
+```c
+struct Point {
+    int x;
+    int y;
+};
+
+int add_points(struct Point p1, struct Point p2) {
+    return p1.x + p2.x + p1.y + p2.y;
+}
+```
diff --git a/tests/fixtures/src/other_langs/c_examples/README.md b/tests/fixtures/src/other_langs/c_examples/README.md
new file mode 100644
index 0000000..0fffd84
--- /dev/null
+++ b/tests/fixtures/src/other_langs/c_examples/README.md
@@ -0,0 +1,16 @@
+# C Examples
+
+This section demonstrates plain C code validation at depth level 2.
+
+Standard C code can be validated without FHE annotations:
+
+```c
+typedef struct {
+    int value;
+    char label[32];
+} Item;
+
+int compare_items(Item a, Item b) {
+    return a.value - b.value;
+}
+```
diff --git a/tests/fixtures/src/c_example.md b/tests/fixtures/src/other_langs/c_examples/c_example.md
similarity index 100%
rename from tests/fixtures/src/c_example.md
rename to tests/fixtures/src/other_langs/c_examples/c_example.md
diff --git a/tests/fixtures/src/other_langs/ts_examples/README.md b/tests/fixtures/src/other_langs/ts_examples/README.md
new file mode 100644
index 0000000..74246bf
--- /dev/null
+++ b/tests/fixtures/src/other_langs/ts_examples/README.md
@@ -0,0 +1,18 @@
+# TypeScript Examples
+
+This section demonstrates TypeScript code validation at depth level 2.
+
+TypeScript type checking works seamlessly with the preprocessor:
+
+```typescript
+interface Config {
+    host: string;
+    port: number;
+    secure?: boolean;
+}
+
+function getUrl(config: Config): string {
+    const protocol = config.secure ? 'https' : 'http';
+    return `${protocol}://${config.host}:${config.port}`;
+}
+```
diff --git a/tests/fixtures/src/typescript_example.md b/tests/fixtures/src/other_langs/ts_examples/typescript_example.md
similarity index 100%
rename from tests/fixtures/src/typescript_example.md
rename to tests/fixtures/src/other_langs/ts_examples/typescript_example.md
diff --git a/tests/fixtures/src/parasol_examples/README.md b/tests/fixtures/src/parasol_examples/README.md
new file mode 100644
index 0000000..451989d
--- /dev/null
+++ b/tests/fixtures/src/parasol_examples/README.md
@@ -0,0 +1,13 @@
+# Parasol C Examples
+
+This section demonstrates Parasol C code validation at depth level 1.
+
+The following examples show how to write FHE programs using Parasol C.
+
+Here is a simple function to test code validation at this depth:
+
+```parasol-c
+[[clang::fhe_program]] uint8_t simple_add(uint8_t x, uint8_t y) {
+    return x + y;
+}
+```
diff --git a/tests/fixtures/src/basic.md b/tests/fixtures/src/parasol_examples/basic.md
similarity index 100%
rename from tests/fixtures/src/basic.md
rename to tests/fixtures/src/parasol_examples/basic.md
diff --git a/tests/fixtures/src/propagate.md b/tests/fixtures/src/parasol_examples/propagate.md
similarity index 100%
rename from tests/fixtures/src/propagate.md
rename to tests/fixtures/src/parasol_examples/propagate.md