diff --git a/wordfreq/__init__.py b/wordfreq/__init__.py
index a895fbc..800fcee 100644
--- a/wordfreq/__init__.py
+++ b/wordfreq/__init__.py
@@ -247,13 +247,14 @@ def word_frequency(word, lang, wordlist='combined', default=0.):
     """
     Get the frequency of `word` in the language with code `lang`, from the
     specified `wordlist`. The default wordlist is 'combined', built from
-    whichever of these four sources have sufficient data for the language:
+    whichever of these five sources have sufficient data for the language:
 
       - Full text of Wikipedia
       - A sample of 72 million tweets collected from Twitter in 2014,
         divided roughly into languages using automatic language detection
       - Frequencies extracted from OpenSubtitles
       - The Leeds Internet Corpus
+      - Google Books Ngrams and Google Books Syntactic Ngrams
 
     Another available wordlist is 'twitter', which uses only the data from
     Twitter.