diff --git a/book.bib b/book.bib
index 344faf5..03246fc 100644
--- a/book.bib
+++ b/book.bib
@@ -202,7 +202,7 @@ @article{Snijders2010margin
 year = {2010}
 }
 
-@Manual{,
+@Manual{R-readxl,
   title = {readxl: Read Excel Files},
   author = {Hadley Wickham and Jennifer Bryan},
   year = {2023},