diff --git a/crates/libcalibre/src/library.rs b/crates/libcalibre/src/library.rs
index 87a680d7..ddd29d2c 100644
--- a/crates/libcalibre/src/library.rs
+++ b/crates/libcalibre/src/library.rs
@@ -776,7 +776,8 @@ impl Library {
         value: Option<CustomValue>,
     ) -> Result<(), CalibreError> {
         let column = custom_columns::get_column(&mut self.conn, column_id)?;
-        custom_columns::set_value(&mut self.conn, &column, book_id, value)
+        self.conn
+            .transaction(|conn| set_custom_value_and_touch(conn, &column, book_id, value))
     }
 
     /// One column's values for many books at once. Books with no stored
@@ -826,12 +827,9 @@ impl Library {
         is_read: bool,
     ) -> Result<(), CalibreError> {
         let column = self.get_or_create_read_state_column()?;
-        custom_columns::set_value(
-            &mut self.conn,
-            &column,
-            book_id,
-            Some(CustomValue::Bool(is_read)),
-        )
+        self.conn.transaction(|conn| {
+            set_custom_value_and_touch(conn, &column, book_id, Some(CustomValue::Bool(is_read)))
+        })
     }
 
     pub fn batch_get_read_states(
@@ -1005,6 +1003,23 @@ impl Library {
     }
 }
 
+/// Compares the stored value before and after the write, so no-op edits and
+/// values `set_value` normalises to what's already stored leave
+/// `last_modified` alone, as Calibre does.
+fn set_custom_value_and_touch(
+    conn: &mut SqliteConnection,
+    column: &CustomColumn,
+    book_id: BookId,
+    value: Option<CustomValue>,
+) -> Result<(), CalibreError> {
+    let before = custom_columns::get_value(conn, column, book_id)?;
+    custom_columns::set_value(conn, column, book_id, value)?;
+    if custom_columns::get_value(conn, column, book_id)? != before {
+        book_queries::touch(conn, book_id)?;
+    }
+    Ok(())
+}
+
 // =============================================================================
 // Metadata OPF generation (ported from calibre_client.rs)
 // =============================================================================
diff --git a/crates/libcalibre/tests/last_modified_test.rs b/crates/libcalibre/tests/last_modified_test.rs
new file mode 100644
index 00000000..690403a9
--- /dev/null
+++ b/crates/libcalibre/tests/last_modified_test.rs
@@ -0,0 +1,91 @@
+// Regression tests for https://github.com/everydaythingssoftware/citadel/issues/157:
+// metadata edits must bump `books.last_modified` only when a value actually changes,
+// matching Calibre's `Cache.set_field()` -> `_update_last_modified(dirtied)`.
+mod common;
+
+use common::{setup_with_library, standard_test_book};
+use libcalibre::{BookId, CustomColumnKind, CustomColumnSpec, CustomValue, Library};
+
+fn add_book(lib: &mut Library) -> BookId {
+    lib.add_book(standard_test_book()).unwrap().id
+}
+
+fn last_modified(lib: &mut Library, book: BookId) -> chrono::NaiveDateTime {
+    lib.get_book(book).unwrap().updated_at
+}
+
+fn add_bool_column(lib: &mut Library) -> i32 {
+    lib.create_custom_column(CustomColumnSpec {
+        label: "finished".to_string(),
+        name: "Finished".to_string(),
+        kind: CustomColumnKind::Bool,
+        is_multiple: false,
+        enum_values: vec![],
+        display: None,
+    })
+    .unwrap()
+    .id
+}
+
+#[test]
+fn test_set_custom_value_touches_last_modified() {
+    let (_temp, mut lib) = setup_with_library();
+    let book = add_book(&mut lib);
+    let col = add_bool_column(&mut lib);
+
+    let before = last_modified(&mut lib, book);
+
+    lib.set_custom_value(book, col, Some(CustomValue::Bool(true)))
+        .unwrap();
+
+    let after = last_modified(&mut lib, book);
+    assert!(
+        after > before,
+        "last_modified must advance after a custom-value edit (before={before:?}, after={after:?})"
+    );
+}
+
+#[test]
+fn test_set_book_read_state_touches_last_modified() {
+    let (_temp, mut lib) = setup_with_library();
+    let book = add_book(&mut lib);
+
+    let before = last_modified(&mut lib, book);
+
+    lib.set_book_read_state(book, true).unwrap();
+
+    let after = last_modified(&mut lib, book);
+    assert!(
+        after > before,
+        "last_modified must advance after a read-state edit (before={before:?}, after={after:?})"
+    );
+}
+
+#[test]
+fn test_unchanged_custom_value_leaves_last_modified() {
+    let (_temp, mut lib) = setup_with_library();
+    let book = add_book(&mut lib);
+    let col = add_bool_column(&mut lib);
+    lib.set_custom_value(book, col, Some(CustomValue::Bool(true)))
+        .unwrap();
+
+    let before = last_modified(&mut lib, book);
+
+    lib.set_custom_value(book, col, Some(CustomValue::Bool(true)))
+        .unwrap();
+
+    assert_eq!(last_modified(&mut lib, book), before);
+}
+
+#[test]
+fn test_unchanged_read_state_leaves_last_modified() {
+    let (_temp, mut lib) = setup_with_library();
+    let book = add_book(&mut lib);
+    lib.set_book_read_state(book, true).unwrap();
+
+    let before = last_modified(&mut lib, book);
+
+    lib.set_book_read_state(book, true).unwrap();
+
+    assert_eq!(last_modified(&mut lib, book), before);
+}