Added BETA books. (#5233)

books/free-programming-books.md

@@ -1064,6 +1064,10 @@ Kerridge (PDF) (email address *requested*, not required)
 ### BETA
 
 * [Object-Oriented Programming in the BETA Programming Language](https://beta.cs.au.dk/Books/) - Ole Lehrmann Madsen, Birger Møller-Pedersen, Kristen Nygaard
+* [MIA 90-02: BETA Compiler - Reference Manual](https://beta.cs.au.dk/Manuals/latest/compiler/index.html) - Mjølner Informatics
+* [MIA 94-26: BETA Language Introduction - Tutorial](https://beta.cs.au.dk/Manuals/latest/beta-intro/index.html) - Mjølner Informatics
+* [MIA 99-41: BETA Language Modifications - Reference Manual](https://beta.cs.au.dk/Manuals/latest/beta/beta-index.html) - Mjølner Informatics
+* [MIA 99-42: The Fragment System: Further Specification](https://beta.cs.au.dk/Manuals/latest/beta/fragment.html) - Mjølner Informatics 
 
 
 ### Blazor