diff --git a/external/book b/external/book
index 2e55fd4630de30ecf86e5c59f89285e5786762ba..2a1d135ccd10247954ba4cc5f870580a76b4530f 160000
--- a/external/book
+++ b/external/book
@@ -1 +1 @@
-Subproject commit 2e55fd4630de30ecf86e5c59f89285e5786762ba
+Subproject commit 2a1d135ccd10247954ba4cc5f870580a76b4530f