An end is simply a new beginning
This may be the end of the book, but it is also the continuation of your journey into software architecture and design. I hope you found this to be a refreshing view of design patterns and how to design SOLID apps.
Depending on your goals and current situation, you may want to explore one or more application-scale design patterns in more depth, start your next personal project, start a business, apply for a new job, or all of those. No matter your goals, keep in mind that designing software is technical but also an art. There is rarely one way to implement a feature but multiple acceptable ways. Every decision has trade-offs, and experience is your best friend, so keep programming, learn from your mistakes, become better, and continue. The path to mastery is a continuous learning loop. Remember that we are all born knowing next to nothing, so not knowing something is expected; we need to learn. Please ask questions, read, experiment, learn, and...