VoiceXML is a topic that I am currently researching in support of developing a business strategy for Unmesh Laddha's company Thinking Minds, Inc.
and our team's resources
for end-to-end support for VoiceXML systems. Among the best resources I've found are:
Additional resources of interest include IBM's AlphaWorks VoiceXML resources
(IBM is one of the major contributors to W3C's VoiceXML specification), Websphere's Voice Server page
and World of VoiceXML
, which is a personal page maintained by Ken Rehor
One of the most highly regarded books on the subject is VoiceXML: Professional Developer's Guide with CDROM by Chetan Sharma and Jeff Kunins. This book has received consistent praise and is up-to-date (it uses the VoiceXML 2.0 specification as reference).