diff --git a/src/book/mod.rs b/src/book/mod.rs
index 38ffbc0b..92b409b7 100644
--- a/src/book/mod.rs
+++ b/src/book/mod.rs
@@ -47,6 +47,16 @@ impl MDBook {
         let book_root = book_root.into();
         let config_location = book_root.join("book.toml");
 
+        // the book.json file is no longer used, so we should emit a warning to
+        // let people know to migrate to book.toml
+        if book_root.join("book.json").exists() {
+            warn!("It appears you are still using book.json for configuration.");
+            warn!("This format is no longer used, so you should migrate to the");
+            warn!("book.toml format.");
+            warn!("Check the user guide for migration information:");
+            warn!("\thttps://rust-lang-nursery.github.io/mdBook/format/config.html");
+        }
+
         let config = if config_location.exists() {
             debug!("[*] Loading config from {}", config_location.display());
             Config::from_disk(&config_location)?