I respectfully disagree - the Library Data book is useful, but it isn't really targeted at newcomers and it doesn't provide much guidance on how to USE the information contained within it in play. I'm thinking of a book that provides high-level advice on how to adapt different campaign models (free traders, mercenaries, military personnel, pirates, secret agents, bounty hunters, explorers, etc) in different periods of history (e.g. Milieu 0, The Classic Era, the Civil War / Hard Times Era, the New Era, the Fourth Imperium, etc). I don't think that something like this has ever been attempted - it's like the introduction to the Spinward Marches book on steroids. Ideally, it would focus on practical advice intended to help GMs answer the question "how do I kick off my first campaign in the OTU?".
DonM's integrated timeline is an impressive work that I've been known to consult on occasion, but it would be very intimidating to anybody who isn't already somewhat familiar with the OTU. It actually contains too much detail for newbies - it would be hard for a newbie to work out which events listed in the timeline have importance and which are merely incidental detail.
The best thing for newcomers is us the community, taking the time to help. My experience with the OTU is, to paraphrase Clausewitz on battle plans, it doesn't survive contact with the game table; just about every OTU I've played in has subtle tweaks by the GM and with how the players that are playing it, changes it.
Considering the breadth of the Imperium in time and size, that is a multi-volume history at least, and the only authoritative writer to write it would be Marc. Otherwise it's a minefield, which I'll bet Matt has little desire to open himself up for a rasher of abuse that would come from even attempting such a project.
Giving it some thought and looking at some of the newer games in the works that seem to have people excited like Nova Praxis or Eclipse Phase, a Trans-humanist supplement would probably sell well and is uncharted territory considering past Traveller work. I have some in my campaign, trans-droyneist actually, it is an interesting subject, throws a nice curveball in there.
Another subject I see come up quite a bit lately are mercenary campaigns, so maybe a supplement with a few mercenary tickets might be popular as well.