Part XX — The Frontier
The book taught you a field's present. This part is its edge — the place where the textbook ends and the arXiv listing begins. You are now equipped to read that listing critically (Part XIV), to build in it (Part XVIII), and to know which of its problems are yours (Ch. 55). Chapter 77 maps the machine era beyond NISQ; Chapter 78 the network future; Chapter 79 the true costs of fault-tolerant algorithms; and Chapter 80 — the last chapter — is the honest ledger of what nobody knows, ending where the book's promise ends and your career begins: at the door of a research laboratory.