Ant´s own resource page lists a lot of links, books and articles. Here are some more.
Ant: The Definitive Guide by Jesse E. Tilly and Eric M. Burke published by O'Reilly - http://www.oreilly.com/catalog/anttdg/
Java Tools for Extreme Programming: Mastering Open Source Tools, Including Ant, JUnit, and Cactus by Richard Hightower, Nicholas Lesiecki ( ErikHatcher contributed to Ant sections ) published by John Wiley & Sons, Inc. - http://www.wiley.com/cda/product/0,,047120708X,00.html
Appendix E of Java Development with Ant is included in the Ant 1.5.1 distribution
Ant 1.5.1 Core Task Examples Quick Reference: http://www.discursive.com/articles/2003/01/ant_core_ref.html