I think there has been something of an informal conspiracy of silence about the contents of the Castle & Crusade Society's famous 1970-1972 fanzine Domesday Book over the years. I suspect this has been driven by a perfectly natural tendency on the part of collectors to create a mystique about rarities in their possession - though really, I'm not pointing fingers here, and even if I were, I'd probably have to start by pointing at myself. People are also starting to get serious about the history of D&D, however, and this secrecy really is doing the historical community a disservice. I think we need to shatter a few myths and shed a bit of light here. I spent the better part of five years trying to assemble enough evidence to be able to say what really happened in the early history of role-playing games, and it was a near-constant process of discovering that widely-held beliefs are inconsistent with documentary evidence.
So, to kick off with some heresy, I don't think the Domesday Book is very important as a historical resource.
Now, this is all relative, of course. There is some useful information in the Domesday Book about how relationships between key people developed and so on. But the articles that appear there are not, as many seem to believe, unique windows into the history of fantasy gaming, either into Chainmail or Dungeons & Dragons. It is certainly not the first place where information about Blackmoor appeared: that would be Corner of the Table, and the Blackmoor Gazette and Rumormonger. The famous "Facts about Blackmoor" article in Domesday Book #13 was in fact reprinted, more or less verbatim, in a later (and far more widely available) source, Arneson's First Fantasy Campaign - this is something that people have conjectured about, I see from a web search, but not something people with the materials in hand have confirmed or denied. It is not the first place where the LGTSA medieval miniature rules from #5 appeared either (that would be Panzerfaust), and in fact, the same rules appeared more or less verbatim in another fanzine (the Spartan International Monthly) shortly after they appeared in the Domesday Book, too.
So aside from "Facts about Blackmoor" and the LGTSA Medieval rules, where exactly does the historical significance of the Domesday Book lie? The article on the Great Kingdom in DB #9 outlines the basics of the campaign world that encompassed Blackmoor and Greyhawk, it's true, but in the article you won't find even a hint of any fantasy dimension, and only scant information about a (failed) attempt to start a wider PBM game in the C&CS. There really aren't any other articles about fantasy gaming in there (the exception that proves the rule is Rick Crane's article in DB#11 complaining about the lack of fantasy content) - there's no smoking gun that proves that Chainmail would have a fantasy supplement prior to its publication. There are other fanzines, however, that provide a far greater insight into the subsequent evolution of Chainmail: the IFW's monthly has a couple of important articles that show how the system was altered to get to its second edition, many of which have important implications for the history of D&D.
I think we've put the Domesday Book on a pedestal here a bit unfairly. Very few of these issues could be said to move the ball forward on the development of role-playing games, and those that do are not the only source for their key contents.
P.S. - Since the apparent scarcity of these rules has been a real impediment to scholarship, it is worth pointing out that there in fact three issues of the Domesday Book (#5, #7 and #9) in the Bowling Green State University library. They were acquired as part of the Hoosier Archives of postal Diplomacy zines. Walt Buchanan asked Gygax to photocopy those three issues from his collection because of their Diplomacy content (#5 and #7 contain Dippy variants). There are some potentially interesting things added to the margins, as these were personal copies. If you're interested in getting more than just a cover scan of these, this library would probably be a good start. Here's the link: