diff --git a/external/book b/external/book
index 9e9fa8e04c221a54cfa83f017bb507bddf428615..1685f671181d37f5d9701a85bd925c34b8fef4bb 160000
--- a/external/book
+++ b/external/book
@@ -1 +1 @@
-Subproject commit 9e9fa8e04c221a54cfa83f017bb507bddf428615
+Subproject commit 1685f671181d37f5d9701a85bd925c34b8fef4bb