The author of this book has a vast experience of developing computer systems and in this book he shares with you many different ideas that make developing and (particularly) redeveloping computer systems much easier. A very useful guide for those working in program development who don't yet have enough experience to have worked out all of these shortcuts for themselves.
Over a period of many years of developing computer systems you gradually learn how much that system requirements are likely to change over time, how following certain conventions makes it easier to apply those changes, and lots of other shortcuts for redeveloping computer systems. In this book the author shares his extensive experience of the development/redevelopment process with you so that you will learn lots of ideas that you can apply during the original development process that will make the redevelopment process to come much more straightforward. In short he tells you how to write programs that will be easy for anyone and not just the original author to maintain but uses a more readable approach in doing so.
The entire development process is covered progressively through the book with an example system that is complex enough to illustrate the techniques being discussed but simple enough to enable the whole system to be included being used to show the application of the ideas presented. Every so often the author also includes an analogous situation (most are sport related) that make the concepts both easier to understand and which make the book much more enjoyable to read.