Why we still write runbooks
Every incident we have handled faster than expected had one thing in common: someone had written down what to do before it happened.
A runbook does not need to be long. A page with the symptoms, the first three commands to run and the person to call is usually enough. We keep ours next to the code and review them during every release.
The best test of a runbook is to hand it to a colleague who has never touched the system and watch them follow it. Whatever they ask about is what the document is missing.