This update was motivated by the latest DD5 Supporters; Highbowf, Sean M. Kelly, Dan Cullinan, and Bill Lackey. Great to have you along!
Delving & Exploration is DD's Wilderness & Underworld Adventures analog (notionally "the ref's booklet"). The 3LBBs have an undeniable charm but lack the kind of organisation that modern readers expect. However, getting DD5's book 2 better organised is a vexing process as there is no "right" answer, and every variation simply favours one priority over others.
After years of changing it up many times, here's how I now think it will shape up:
Preface
Contents
Fantastic Medieval Wargames
Organisation
Referee's Intro
Preparing for the Campaign
The Known World
Wilderness Preparation
Town Preparation
Underworld Preparation
Moderating the Adventure
Harzards
Underworld Adventures
Monster Reaction to Players
Fantastic Medieval 1:1 Scale Battles
Wilderness Adventures
Moderating the Campaign
Player Characters
Non-Player Characters
The Campaign Turn
Far Horizons
Other Worlds
Artifacts
There are still a few details to be resolved, but I think this works pretty well as it addresses the referee's likely workflow.
Next steps are to move slabs of text around, starting from the top and working my way down...
