To get you in the mood for ultraviolence... er, playing by e-mail, you might try cracking open a book or checking out the remains of previous games.
Prayer for the Technocrats: Marcus' novella about the Caal Invasion, the collapse of the Federation, and the birth of the Holy Terran Empire. No, you don't have to read it to know what's happening, but it's a quick and really good read. You can also buy a hard copy of it here.
Born Under a Gray Sky: Taking place six months after the events in Prayer for the Technocrats, this is the story of the Ascension War between the dying Federation and the fledgling Holy Terran Empire. Basically, all the events that happened a few years before the start of this miniseries.
Rage Against the Dying of the Light: Season 4.3. Taking place over thiry years before this miniseries, it chronicles the story of the Federation's Third Civil War.
The Middle Kingdom: Season 8, chronicling an alternate-history timeline where the Eastern Bloc conquered almost all of human space and the Caal Invasion never happened.
Earth Fleet: Martin's magnum opus - his website dedicated to detailing and categorizing the all of Earth Fleet's warships in Season 4.3. My God... it's beautiful!
The TwikI: A Tech Infantry wiki site explaining more than anyone ever wanted to know about the people, places, and technology of the TI universe.
TV Tropes: The Tech Infantry web page that Martin developed on this website. It's a great analysis of the entire series.
If anyone has any of their original results or additional info from Season 4.3 & 8, please send it to me. I'm compiling a printed copy of the other PBeM stories.
Sci-Fi Literature & Movies
Although these aren't technically TI, they are the inspiration behind it.
Starship Troopers by Robert Heinlein. This explains the Tech Infantry (in that universe, the Mobile Infantry) very well and is the foundation for the entire universe. The movie is good to get the propaganda down, but otherwise, it's iffy for our purposes.
Vampire: The Masquerade, Werewolf: The Armageddon, and Mage: The Ascension (1st Edition) by White Wolf. These were the three rule books originally used to run the game.
Armor and Vampire$ by John Steakley. Armor explains the Bug War very well, plus gives you a good insight into the trooper on the ground. Vampire$ is a good book to get the concept of a Crusader team down.
The Man Who Never Missed, Matadora, The Machivelli Interface, and the Albino Knife by Steve Perry (not the singer from Survivor). This explains the Resistance as well as the general feel of the universe.
The Bio Of A Space Tyrant trilogy by Piers Anthony. This explains the Digital Gate concept.
Aliens (the second movie, not the first one). Not only the definitive military sci-fi movie, but gives you a good idea of what fighting the bugs would be like.
Babylon 5 (the TV series). A lot of the technology and concepts are based off this show.
Chung Kuo by David Wingrove. VERY cool sci-fi about a Chinese-controlled Earth - lots of political intrigue. This is where we get the Middle Kingdom concept from.