diff --git a/free-programming-books.md b/free-programming-books.md
index 83463ed6626e056c7520bc99043bd193ba77e044..b1bc5c519852c7eb3c9a31c27a1f4415d8e0bc41 100644
--- a/free-programming-books.md
+++ b/free-programming-books.md
@@ -841,8 +841,8 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### APL
 
-* [A Practical Introduction to APL1 & APL2](http://aplwiki.com/BooksAndPublications#A_Practical_Introduction_to_APL1_.26_APL2)
-* [A Practical Introduction to APL3 & APL4](http://aplwiki.com/BooksAndPublications#A_Practical_Introduction_to_APL3_.26_APL4)
+* [A Practical Introduction to APL1 & APL2](http://robertson.uk.net) - Graeme Donald Robertson (PDF)
+* [A Practical Introduction to APL3 & APL4](http://robertson.uk.net) - Graeme Donald Robertson (PDF)
 * [Mastering Dyalog APL](http://www.dyalog.com/mastering-dyalog-apl.htm) (PDF)