Most useful EHC documents?

A lot of EHC content is created and made available to the field and customers.

It is difficult to measure just how useful or effective the various documents are.

Depending on your job role, which document(s) do YOU find most valuable and help your day-to-day EHC activities? And Why?

• Reference Architecture
• Concepts and Architecture Gudie
• Administration Guide
• Operations Guide
• Security Guide
• Guardrails
• Release Notes
• Other?

This is a totally unofficial poll, just looking to see what the discussion might uncover. Please comment below ...