Software

Two books that I found helpful when I was starting Domino administration are:

Dahl, Andrew. 1996. Lotus Notes 4 Administrator's Survival Guide. Sams Publishing.

Lamb, John and Peter Lew. 1998. Lotus Notes & Domino Network Design. McGraw-Hill.
The Administrator's Survival Guide is good for the day-to-day "how do I manage users" questions. The Network Design book is good for more theoretical topics like Domino domain layout. You may not need to know about advanced domain management now, but you'll eventually face a replication and routing issue that is made clear by understanding the theory behind it.

The web is also a great source for learning Domino. Just browsing through the Iris Today white papers at http://www.notes.net you're bound to pick up a tip or two to solve a problem you didn't know you had. You can also find some handy utilities in the Sandbox at Notes.net.

Hope this helps.