The book 'Becoming Fluent' by Richard Roberts and Roger Kreuz explains how adult learners can use their skills and knowledge to master a foreign language. The authors draw on insights from psychology and cognitive science to show that adults can learn a new language more easily than children. Adults have an understanding of their own mental processes and an advantageous grasp of pragmatics, the social use of language. By applying the tools acquired over a lifetime, learning a language can be enjoyable and rewarding for adults.