While Dawn of the Jedi is a rather new invention, for over fifteen years, this prologue by Kevin J. Anderson to Tom Veitch’s Tales of the Jedi comics was the start of the Star Wars universe. The reference guides - The Essential Atlas and Chronology - had stories, there were articles written by Del Rey and LucasFilm Ltd staff - but in terms of easily consumed stories, this was the Genesis of my generation of fans.
Tales of the Jedi: Golden Age of the Sith starts with issue #0: Conquest and Unification. This story features Jedi Apprentice Odan-Urr on a mission for his Master Ooroo. This Jedi Scholar, whining the entire time about how he is not cut out for this mission, teaches the skill of Battle Meditation to Memit Nadill, Jedi advisor to Empress Teta of the Koros System, which would historically be known as the Empress Teta system. That makes these the titular Unification Wars, the wars fought between Empress Teta and the dissidents within her system to unite the worlds there under one common front. What's of note here is that while Teta clearly seems to be a wise and benevolent ruler (as we'll see later), we have yet to see any indication as to what the cause of these wars are. Whether this is a Firefly style war of simple domination or whether the rebels in question did something to require unification, we never know, but ultimately Teta wins the war.
Golden Age of the Sith proper begins with fraternal twins Gav and Jori Daragon, two down-on-their-luck fringers in the Koros System. Feeling that they are out of options to make their way, they steal their parents' ship from the custody of Aarrba the Hutt to use for one last chance as deep space scouts - individuals who risk their lives by entering hyperspace along unplotted coordinates and selling those coordinates to Spacers' Guilds. There's a subplot involved here wherein a merchant lord used a route that Gav and Jori had found, but labeled to be unsafe to use, but that won't become important until later.
I feel it's important here to address the matter of the Sith. Tales of the Jedi, in large part, exists to bring the Sith from their original state as a distant Empire to the Sith that we know today. It doesn't go the whole route, of course - the Rule of Two wasn't introduced in this series, and I'm uncertain as to what year Jedi vs Sith was released - but it takes long steps over the course of the next several miniseries. In Golden Age of the Sith, Emperor Marka Ragnos has just died after a century of unquestioned rule (once he removed the head of Simus, who now functions as a member of the Sith Council, anyway), and the question of his successor is in question. Naga Sadow, who is the fastest to claim the title of Dark Lord and Emperor for himself, feels that the only way for the Sith to survive is to unite the Empire against a common enemy and begin to take new ground through force. His chief opponent, Ludo Kressh, is convinced that Sadow's plan is folly and is sure to guarantee the destruction of the Sith people through war.
The Daragons, led by the Force, though they do not know it, find what in other circumstances would have been a very fortuitous prize indeed: an unimpeded Hyperspace corridor that leads from the Inner Core to the Outer Rim. This corridor from the Koros System to the Imperial throne world of Korriban is one that would rarely be used in the future, owing most likely to the fact that a pathway between the centers of power of both the Sith and the Empire would likely be filled with hyperspace mines and fortified areas for the next 5,000 years. For the purposes of events in the comic's near future, however, the Daragon Trail, as it will come to be known, has become the most important thing in the galaxy.
The rest of the volume is spent following two sequences of events: the reactions in the Koros system to Gav and Jori's actions (both the saurian spacer who blames the Daragons for the loss of his ship and the Hutt who felt both robbed and betrayed by their actions), and Naga Sadow's manipulations of both his human captives and his fellow Sith Lords
For an early Kevin J Anderson story, Golden Age of the Sith requires relatively little in the way of leaps of faith. Once you can believe that the Sith and Jedi both have different powers than the ones that you're familiar with in the modern era, this works as a tale that transforms from a quaint historical anecdote into an epic about a major turning point in galactic history once the Sith discover the Republic.
I'd like to take a moment to talk about the art, not only for this story, but for the entire Tales of the Jedi line. This is an older story, as far as Star Wars is concerned, and certainly art standards for comics have changed since the 90s, especially for publishers with less budget behind them than DC and Marvel. Still, while the art of this series would not be appreciated in the modern day, for a historical story such as this, I actually feel like the art present here is superior. It gives it a sort of gritty feel, as though you're looking at a recollection, rather than watching events as they happen. It also gives a sort of mystical tone to the proceedings that I feel matches very nicely with the Sith sorcery and more fantastical powers exhibited by the Jedi in the Tales line of comics.
As a whole, Tales of the Jedi: Golden Age of the Sith does not tell a complete story, but it was never intended to. This story acts as a massive prequel, comparable to Fellowship of the Ring in its role in the buildup to massive, galaxy-spanning war. The Sith build up for war, not only with the Republic, but with themselves, and with a title like Fall of the Sith Empire following, it's not hard to imagine the results of such a two-front war on the Sith.
My recommendation of Golden Age of the Sith, then, is that you should read it if you intend to read both parts of the Great Hyperspace War. In fact, if any of this appeals to you, my suggestion is to buy the Omnibus Edition of Tales of the Jedi Part 1. This features the whole of the Great Hyperspace War, in addition to several of the succeeding stories one thousand years in this story's future.