Explorer's Guide

From LSWiki

This is a book all Explorers get for free. This particular edition cannot be dropped or destroyed.

It contains many old, in-character articles about earlier iterations of the MUD, that have not been updated in many years, including now-deleted content. There doesn't seem to be a way to remove old articles manually, or at least nobody has done so (a developer probably), so there are some leftover, even, from before the 3D map and the many Aedaris regions were introduced. I believe there are even a few from the late 90s.

The Wiki (which you're reading now) is its modern replacement.