diff --git a/book.toml b/book.toml
index 43952d4..b5d40d6 100644
--- a/book.toml
+++ b/book.toml
@@ -5,6 +5,9 @@ multilingual = false
 src = "src"
 title = "Monads in Rust"
 
+[rust]
+edition = "2021"
+
 [build]
 build-dir = "book"
 create-missing = false