The most recent editions of the ADH (2000 and 2002) had to be published in
two volumes because there were simply too many pages to fit into one volume.
The first volume tends to focus on features that are relevant to all Access
applications, while the second volume tends to focus more on multi-user
features. Most Access developers will want both volumes.
As for which addition to buy, as others have indicated, it depends on what
version of Access you are using, but also on whether you intend to continue
to use that version or to upgrade.
If you're using Access 97, and intend to continue using Access 97, go for
the Access 97 edition. If you're using Access 2000 and intend to continue
using Access 2000, go for the Access 2000 edition. If you're using Access
2000 but intend to upgrade in the not very distant future, or if you're
using Access 2002 or Access 2003, go for the Access 2002 edition. There is
no Access 2003 edition, but the Access 2002 edition will remain very
relevant and useful to anyone using Access 2003.
Check out the web site for the book (
http://www.developershandbook.com/).
Read the paragraph at the end of that page. It explains the reason for and
difference between the two volumes.