Someone has to write the "VFP Combo boxes Bible"

Go to http://www.hentzenwerke.com/ and download the sample chapters from 1001 Things You Wanted to Know About Visual FoxPro

Chapter 5 is all about combos and lists:

Chapter 5: Combos and Lists

Combo and list box basics
List and ListItem collections
When do the events fire?
How do I bind my combo and list boxes?
How do I refer to the items in my combo and list boxes?
What is the difference between DisplayValue and Value?
What's the difference between RowSourceTypes "alias" and "fields"?
How do I make my combo and list boxes point to a particular item?
Quickfill combos
How do I add new items to my combo and list boxes?
How do I filter the items displayed in a second combo or list box based on the selection made in the first?
A word about lookup tables
Generic lookup combos and lists
So what if I want to bind my combo to a value that isn't in the list?
How do I disable individual items in a combo or list?
How do I create a list box with check boxes like the one displayed by Visual FoxPro when I select "View Toolbars" from the menu?
A mover list class
What if I need to display hundreds of items in my combo box?