This is the core of the book. Jose Apraiz Barreiro provides exhaustive data on:
If hardening is making the steel hard, tempering is making it usable. Barreiro points out that as-quenched martensite is brittle as glass. He provides detailed charts showing the loss of hardness versus gain in toughness at increasing tempering temperatures (from 150°C to 600°C). He also warns about specific phenomena like "temper embrittlement" in the 450-550°C range—a critical warning for tool makers. This is the core of the book