Van Richten's Guide
From an in-universe perspective, the Guides are, as their title says, treatises on monster-hunting written by the esteemed Dr. Van Richten, the most prolific and well-educated monster hunter in all of the Demiplane of Dread. At some point, Van Richten disappeared, after which his legacy was taken up by the Weathermay-Foxgrove Twins. In addition to completing and publishing his own unfinished notes - the Guides to the Fiend, Vistani and Witch - they went on to create their own guides, which they published under the same name for legacy purposes and to ensure they would reach their target audience.
On the meta-level, each Guidebook examines a single creature type and expands upon it to make it both a better fit for Ravenloft's purported "Gothic Horror" goal (even if, in practice, it leans closer to Castlevania amongst most people who actually play it), providing an exhaustive examination of tactics that can be used to run that monster or fight that monster, variant "salient abilities", unique weaknesses, lore, and other aspects that flesh them out.
Van Richten wrote and published on his own the following books for the Advanced Dungeons & Dragons system:
- Guide to Vampires
- Guide to Liches
- Guide to Ghosts
- Guide to Werebeasts
- Guide to Fiends
- Guide to the Ancient Dead
- Guide to the Created
- Guide to the Vistani
These books were later reprinted in "collected editions" by the Weathermay Twins, known as the Van Richten's Monster Hunter's Compendiums. The third and last of these volumes also included the otherwise unprinted "Van Richten's Guide to Witches", which, as the name suggests, examines Witches, Warlocks and Hags.
The Weathermay-Foxgrove Twins wrote two guides completely of their own for Dungeons & Dragons 3rd edition which saw print; one for the Walking Dead and a second for the Shadow Fey. A third guidebook, dedicated to "The Mists", which focused on anomalous and eerie entities born from the reality-binding vapors that wreathe the land, in addition to covering Outlanders (humanoids from other worlds brought amongst the ignorant natives of the Demiplane of Dread) and anomalous reality-zones that one might encounter whilst enveloped by the Mists, was completely written up... but, White Wolf lost the license before they could officially print and sell it. The PDF was set free onto the internet, and is now hosted on, amongst other things, the Fraternity of Shadows website.
Looking into adapting content found in these guides to later editions? Note that not everything written by White Wolf & Sword&Sorcery for 3-3.5e was converted entirely everything from the 2nd Edition, and- more intriguingly, the powers Liches are privy to in Guide to the Lich are still, 3.5 canon, such as the salient abilities Liches gain are in Monsters of Faerun, though only a small excerpt. Please note, that these make both liches and vampires essentially epic-level threats, whom are more dangerous depending on their personal holdings and age- a vampire patriarch for example, is essentially a demigod immune to sunlight, and probably only beatable if they gained additional weaknesses over their long-term existence, they can even fully heal themselves once per day through the use of their alternate form ability, which has an additional form of their choosing- essentially their second Castlevania Dracula Form- limited to no CR or cap or anything. They can even cheat themselves into becoming this strong. If you've ever heard of the epic-questline, the Quicksilver Hourglass in 3.5, a quest where you have to stop members of the Union of Eclipses using a deific artifact to age the world to death and rule the remainder, then failure would result in vampires like this popping up all over the material plane. Bullet dodged.