The book explains the process for designing and modernizing software applications, applying Domain-Driven Design methods to all design and development stages. It describes creating business models using canvases and capability maps, gathering business requirements using domain storytelling and visual glossaries, designing the macro architecture using event storming, and designing single services using tactical and API design. It also describes how to involve all development or modernization partners, such as business experts, developers, or customers, in application development in a highly collaborative and engagement-driven process.