You can find comprehensive information on AutoCAD Menus in the AutoCAD help system. Here are instructions for finding the section devoted to Menus.
1. In AutoCAD, choose Help > AutoCAD Help Topics from the menubar
2. Click on the Contents tab in the Help Topics dialog box, then double click on Customization Guide
3. Double click on Part 1 - Customization Reference
4. Double Click on Chapter 4 - Custom Menus. You will see the list of topics. Click on the topic of your choice.
You can also refer to Chapter 20 of Mastering AutoCAD for tutorials on menu customization.