The Mac OS X Address Book usually sorts contacts by first name or last name; however, after updating a system to Snow Leopard or even with an incremental update for Leopard, contacts may appear jumbled and will not sort properly. Usually, some faulty preferences are to blame, but if you do not use Address Book frequently, you may also mistake some interface behavior for faulty sorting.
One thing to keep in mind about Address Book is that it will only sort addresses two ways: alphabetically by first name or by last name. This is a limit of the program. While you can click the "Name" title bar (which usually changes the sorting of lists from ascending to descending, and vice versa), the program does not recognize this as a command to change sorting orders. This might be a little confusing to people who don't regularly use Address Book. The only ways to change the sorting of Address Book are in the program's preferences, so first try resetting them and then try removing the preferences file if the first does not work.
1. Reapply the sorting settings.
Go to the Address Book preferences and in the "General" section choose either "First Name" or "Last Name" under the "Sort By" menu. This will, hopefully, reset the settings and organize you contacts properly.
2. Remove preferences
The address book preferences do not hold many customized options for the program, with LDAP/CardDAV/Exchange accounts being the most configurable items held in the preferences. As such, you can try removing the preference file for Address Book and try relaunching the program, without losing too many settings. The preference file is called "com.apple.AddressBook.plist" and is located in the /username/Library/Preferences/ folder.