The Ant User's Manual[12] at the Ant project homepage[13] (which itself is part of the Apache Project[14]) is the definitive resource for ant.
[12] http://ant.apache.org/manual/
[13] http://ant.apache.org/
[14] http://apache.org/
O'Reilly has also published Ant: The Definitive Guide by Jesse Tilly and Eric M. Burke (ISBN 0596001843), which was the first book on ant that we read.
If you are into Extreme Programming you'll want to check out Extreme Programming with Ant: Building and Deploying Java Applications with JSP, EJB, XSLT, XDoclet, and JUnit by Glenn Niemeyer and Jeremy Poteet, published by SAMS (ISBN 0672325624).
