In addition to this document (Kerio MailServer 6, Administrator's Guide), the following documentation goes hand in hand with Kerio MailServer:
Kerio MailServer 6, Step-By-Step Guide — this document focuses on installation and basic configuration of Kerio MailServer.
Kerio MailServer 6, User's Guide — this manual includes detailed information on setting and using of client applications and the Kerio MailServer's web interface.
Besides the documentation, you can also target various issues by referring to:
Product forum — in this discussion, you can encounter experience and problems of other administrators using the same product. You may find a working solution for your issues here.
Knowledge Base — here you can find a set of articles troubleshooting particular problems.