diff --git a/references.bib b/references.bib
index fed418b..32fb766 100644
--- a/references.bib
+++ b/references.bib
@@ -9,6 +9,14 @@ @book{brinton1882
   year           = {1882}
 }
 
+@book{gates1931,
+  author         = {Gates, William E.},
+  publisher      = {Maya Society Publication 1, Baltimore},
+  title          = {An Outline Dictionary of Maya Glyphs: With a Concordance and Analysis of 
+                    Their Relationships},
+  year           = {1931}
+}
+
 @book{roys1933,
   author         = {Roys, Ralph L.},
   title          = {The book of Chilam Balam of Chumayel},
@@ -23,6 +31,14 @@ @book{vonhagen1944
   year           = {1944}
 }
 
+@book{zimmermann1956,
+  author         = {Zimmermann, G\"unter},
+  publisher      = {De Gruyter Mouton},
+  title          = {Die Hieroglyphen der Maya-Handschriften},
+  year           = {1956},
+  doi            = {https://doi.org/10.1515/9783111588933},
+}
+
 @book{thompson1962catalog,
   author         = {Thompson, John Eric Sidney},
   title          = {A Catalog Of Maya Hieroglyphs},