Our brains tend to deteriorate as we get older. On the other hand wisdom seems to be associated with old age. In The Wisdom Paradox
Elkhonon Goldberg examines this apparent contradiction, and looks at what we can do to get the wisdom rather than the deterioration. He shows that, as well as the holistic/reductionistic distinction, recent work links the right side of the brain to dealing with novelty and the left to recognising patterns. He looks at how the relative importance of these two sides changes as we get older - and at what can be done to influence this.
However, I wouldn't suggest this book if you just want a 'how-to' book for mental fitness as you get older. Firstly, it doesn't just look at the effect of aging - the early chapters look at a wide range of neuroscience (including much of Goldberg's own research). Secondly, Goldberg runs a 'cognitive fitness program', and I felt that his later chapters were as much about promoting this program as about telling readers ways to improve their own mental fitness. Hence this is more of a book for readers who want a wider picture of what happens to our minds as we get older.