I don’t recall ever seeing anything online that was helpful. The vagueness you mention, I think, is due to the variability of ADD, and unless you’re talking about a specific person, you can’t really give specific information.
I got some education on ADD from college (I was a psychology major). I also read the classic Driven to Distraction by Edward Hallowell, which was very helpful in a clinical, overviewy way, and the follow-up Delivered From Distraction.
I think I still have the books. If you want them, drop me an email and I’ll send them to you gratis.
Aside from those books, I suggest talking in depth to your SO, if they’re willing. They have had a lifetime of experience with ADD. They can tell you exactly his/her history with ADD, how they act or feel under its effects, how you can watch out for undesireable behavior, what behavioral, chemical, or other treatments and techniques work for them, etc. Ask questions about their experiences in school, at the workplace, in high-stress situations, casual, friendly get-togethers, that sort of thing. Since there is a genetic component to ADD, ask about immediate family members who have or may have ADD, how it affected them, what worked and didn’t work for them. Ask how non-ADD family members coped.
As much as their ADD will affect you, it’s important to understand its role in your SO’s life, and to be able to discuss it with them. Not only will it give you greater understanding of them and bring you closer, it helps to be able to discuss it calmly when you can. Conversations that happen later, after ADD-related incidents, may not be as relaxed.
From there, you can discuss if your SO wants you involved in her ADD management, and if so, how exactly to go about doing this. Your goal is to help, not enable, nor should you take on too much of the work. Most of the work should be theirs to do. Your role is primarily guidance, supervision, feedback, encouragement.
One last thing: when talking to your SO, keep in mind that it can be a deeply personal and very delicate subject. We’re talking about effects that can make a person feel like a totally incompetent failure for their entire life. It can be difficult to understand, much less relate to that.
Kudos to you for trying to understand ADD, by the way. I think both you and your SO will benefit from this attitude.