Fantasy history is a tricky thing. As a teenager,I remember pouring the timelines and stories of worlds like Dragonlance, Greyhawk, Mystara and Faerun - The inconsistencies were a point of fascination, a sign that the world was not fully 'known'. Studying the history of the setting was a big part of the setting for me.
These days, I have to admit I find it less interesting. Mostly because I am no longer as enchanted by the history of a setting in and of itself, but rather what it brings to the present setting.
1st Maxim: Only tell the history of how the present day came to be.
This is an important maxim when writing history, that I failed to observe for a long time writing the timeline for Erce. It was hopelessly detailed with my own little vignettes of the ancients, but far too little of it told the reader anything about the present day of the setting.
2nd Maxim: Preserve the sense of Mystery.
Writing history with a sense of mystery tends to make for more evocative study Less is often more and nowhere is it more true than setting history. History should be shards and fragments alluringly peaking through the mists of time to hit the present - This is true also for the creator himself - A world mysterious to its own creator is just a lot more fun to create for than one where everything is laid bare.
I enjoy the sense of my own setting history far more when I myself only have sketchy ideas of the Godmakers who inadvertedly wrought cosmic destruction on the world - Or that the psychic wars were fought between two realms who were once one - and both said to have simply disappeared from Erce altogether, practically overnight. I don't know to know what really happened to the race who fought the psychic wars. Or how the godmakers invoked such destruction. Restricting myself as creator to developing in my own mind's eye only what a historian in the present could hope to learn of them is an exercise in writing evocatively and also keeps you grounded in the 1st maxim; only documenting the history that plays a part in defining the present of the setting.
3rd Maxim: Brevity, Brevity, Brevity
Refraining from too much detail tends to come with the benefit of needing to write much less.Often, this is an exercise in restraint more than justification for laziness. So it needs stating that brevity is a virtue in itself when writing a history. The rule of thumb is - players should be able to read it in half an hour and come away with a rough idea of the history of the world. Don't do a 30 pager if 5 pages can do the same. The maxims above tell you how to condense. Following a maxim from Bertrand Russell's "How I Write" should also help:
There are some simple maxims-not perhaps quite so simple as those which my brother-in-law Logan Pearsall Smith offered me-which I think might be commanded to writers of expository prose. First: never use a long word if a short word will do.
This simple maxim really should be a golden rule of thumb for all prose writers. In my setting pack, I go over every paragraph I wrote (usually a few days later when I can distance myself better and read it anew) and see if I can cut out words or rephrase sentences to say the same thing with fewer words. Trust me, your readers will love you for it. Being able to present your information in 2 pages instead of 3 simply makes a presentation so much more accessible and digestible.