To quickly introduce such a liason to the concepts of software development, I'd recommend Facts and Fallacies of Software Engineering - small, easy to read book. References provided in each section allow for a deeper topic study if needed. Recommended reading for those interested in getting software right. For a more thorough study, one probably can't go wrong with Code Complete - fundamental and well presented overview of professional software development.
For liason role it would be helpful to also understand concepts of software quality assurance. As far as I can tell, our colleagues at SQA.SE recommend Lessons Learned in Software Testing: "...it covers a wide-enough range of topics that it makes for great conversation around what we do, and the thinking behind why we do it that way..."
- As a supplementary reading in case if this guy is willing to actively participate in design discussions (I wouldn't bet that this is a good idea though) I would suggest Applying UML and Patterns - an excellent tutorial on object-oriented analysis and design, highly recommended. POS system case study alone makes it worth reading the book. Another thing I like in this book is nicely described and explained walkthrough of a reasonably productive iterative development process. Note title reference to UML and patterns is rather misleading, subtitle is much better fit for contents: "An Introduction to Object-Oriented Analysis and Design and Iterative Development"