Update README.md

Author: wangbin579

README.md

@@ -54,10 +54,12 @@ If you find any errata in the book, [please open a new issue](https://github.com
 
 [Computer Science Basics](https://enhancedformysql.github.io/tech-explorer-hub/reading/computer-basics/index.html)
 
-[High Performance MySQL](https://enhancedformysql.github.io/tech-explorer-hub/reading/database/mysql.html)
+[Explore MySQL: A Beginner’s Journey into Database Internals](https://wangbin579.gumroad.com/l/mysql_course)
 
 [Architecture of a Database System](https://enhancedformysql.github.io/tech-explorer-hub/reading/database/database_architecture.html)
 
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 ### Access the Book on GitHub Pages
 
 [Online URL](https://enhancedformysql.github.io/The-Art-of-Problem-Solving-in-Software-Engineering_How-to-Make-MySQL-Better/)