An address book lets you store your commonly used email addresses. You can call up the address book from the File menu, the address book button on the Main window, or the address book button in the Composer window.
You can choose between different types of address books in the Appearance/Addressbook section in KMail's Configure KMail... dialog.
The default address book is KAddressBook, which has its own documentation.
To add addresses to the address book, type an email address into the lower section of the window and hit the Add button. The address will then appear on top of the list (the next time you open the address book it will have its place according to the alphabetical order). To remove an address, select it from the list and hit the Remove button.
Addresses can also be added to the address book from the Mail Reader window by right mouse button on the field and choosing a different completion mode. clicking on an email address in the To: or From: fields and selecting Add to address book.