"AutoCAD 2000 VBA Programmers Reference" by Joe Sutphin, ISBN 1861002564, published by Wrox Press (http://www.wrox.com), available at amazon.com or bol.com
"Mastering AutoCAD VBA" by Marion Cottingham, ISBN 0782128718, published by Sybex, available at amazon.com and bookpool.com
The first title is currently out of stock but i can recommend the second one for vba beginners.
Niels