diff --git a/src/bibapi/sru.py b/src/bibapi/sru.py
index 8cdda6a..1e571a0 100644
--- a/src/bibapi/sru.py
+++ b/src/bibapi/sru.py
@@ -542,7 +542,7 @@ class Api:
         sr = parse_search_retrieve_response(resp.content)
         return sr.records
 
-    def getBooks(self, query_args: Iterable[str]) -> List[BookData]:
+    def getBooks(self, query_args: Union[Iterable[str], str]) -> List[BookData]:
         records: List[Record] = self.get(query_args)
         # Avoid printing on hot paths; rely on logger if needed
         books: List[BookData] = []