I'm ashamed I don't have an answer for this: a client is a CEO who finds himself, after a successful career in the domain, in charge of a company developing software in that domain. Naturally, he is more familiar with traditional project management and when he thinks about what worked for him in the past, it's what you and I would call "waterfall process." He sees that there are problems with this and when I explained to him that it's now widely agreed that iterative, incremental approaches are more effective, he was receptive and wanted to learn more.
What should he read? Even the highest-level books I know to be excellent (authors like Jim Highsmith, Mary & Tom Poppendieck, DeMarco & Lister) are really for technical managers. What this guy needs is, like, a Harvard Business Review study. He doesn't need to know the details of XP versus Scrum, he needs to see that abandoning the concept of "perfect requirements" can lead to business success.