I don't know that there are any resources specifically for the Diplomatic Victory. However, what you'll need to do is research the Alliances technology in the Civilization tree, and you'll need to be at least as strong as the nations that you want to ally with, because the AI Factions will only rarely ally with anyone weaker than they are. It will also help if you have positive diplomatic modifiers, such as the Diplomat sovereign profession, and give the factions you wish to ally with the occasional gift to help move relations towards friendly.
Other things that can help with improving relations are the mutually-beneficial treaties (like the Trade Treaty), common wars (but don't make a common enemy if it and your desired ally are both of the same alignment - the 'you attacked a fellow _' penalty tends to more than counter the 'we have common enemies' bonus). Nonaggression pacts can help keep the peace while you butter up someone to become an ally, but don't forget that you really should be the stronger partner in the alliance if you want the AI to accept the deal.
The last thing to remember about the Diplomatic Victory is that you don't need everyone allied to you to also be allied with one another (it'll help, because otherwise there's a decent chance that wars will flare up between your allies, which may force you to break at least one alliance, which will then really tick that former ally off), and you don't need to be allied to everyone. Once you have your desired set of allies, wipe everyone else out and you've won a "Diplomatic Victory". This is, in fact, the most likely way you'll win a diplomatic victory, because usually there's one or two hold-outs who just won't join the alliance for love or money despite being faced with a 10-1 power disparity if you factored in the strength of all your allies with yours and compared it to their strength (this is assuming that there are ~10 factions with roughly equal strength, or an equivalent situation where you and your allies make up ~90% of the total faction power in the game, which isn't that hard to imagine if you and your allies have been fighting and winning wars against non-allies).
As for why it's the red-headed stepchild - that's because it tends to be a pain in the butt to get the AI factions to accept an alliance treaty, even if you have a significant strength advantage over them, and because the Alliance Treaty is only available through a technology which you're only likely to research at a fairly late point in the game, which means that relations have had a lot of time to degrade due to close borders, the occasional war, competition over expansion space, refusals of stupid trades and Tribute Treaties, or whatever the other diplomatic modifiers come from. Then there's also the issue of not being able to back out of any of the treaties in the game until said treaty expires or an event pops up that allows you to either honor your agreement or break it. As a warning, random events such as Season of Blood (all factions are forced to declare war upon anyone with whom they are not allied) can make it very, very difficult to pull off a diplomatic victory after the event occurs.