The first book is VoiceXML: 10 Projects to Voice Enable Your Web Site (affiliate link), by Mark Miller

VoiceXML is rapidly becoming the de facto language standard for handling the transition from visual Web browsing to synthesized text-to-speech information access. This book provides Web designers, database managers, and systems designers with all the tools and templates they need to accomplish this transition in the least amount of time and at the lowest possible cost. Mark Miller walks readers step-by-step through ten real world applications-from statement of the problem through code examination, error checking, and deployment-to clearly demonstrate how to use currently available tools, techniques, and strategies for building Internet voice applications. Accompanying each applications project is a telephone number that allows readers to listen to, and interact with, an actual working example of the application.

Definitive VoiceXML bridges the gap between enterprise computing and telephony engineering, demonstrating exactly how to build new enterprise-class voice applications and voice-enable existing applications. It's the first book that delivers the depth and breadth of knowledge needed by both enterprise and telephony developers, thoroughly addressing the unique challenges of building voice systems for business.

Both books are excellent and I really can't decide between one of them so I bought them both. Two great reference for IVR VoiceXML application development.