Like the others I have a better than average memory, but I don't have financial resources to get a bunch of extra material, so most of my stuff comes from Wikipedia and then from other websites.
One really helpful one for me was one where there is a visualization of the sides and their positions in Chancellorsville, including how the armies moved; this helped me to pinpoint how I wanted the battles, including the alternate stuff after that battle, to go.
Since I'm legally blind and have other minor physical handicaps, scene-setting isn't as visual with me as it is for others; I rely more on dialogue than on peoples' appearances.
So, I start my alternate Battle of Chancellorsville in "Brotherhood and Baseball" with a soldier teasing another (who had apparently been singing), "Think the pathetic vibratiosn of your vocal chords will war off Rebs?" We the learn through dialogue that they are in Reynolds' group and have only been marching the whole time, then word of Hooker's concussion hits and Reynolds rides through urging the troops onward; we see what happened to the general in the ATL, etc.
I found Wikipedia most helpful for who was where and when as far as terms of office, etc.. By the time I get into 1910s America and have President Underwood doing stuff, including the early Civil Rights movement, I had to figure out what differences might have occurred in 50 years. There, I relied on that memory and some small scenes I'd written; I don't write a book linearly most of the time, instead I write as I imagine scenes.
Now, if I'm doing a timeline, it's mostly narrative, sort of like a history teacher in an entertaining lecture. There, it's mostly Wikipecdia and maybe a few other sources because I'm not trying to sell anything. I keep it plausible but I also have too many other thigns going on to pour a whole lot of effort into it.
I, too, would get bogged down in note taking anyway. My problem at times is I have too many choices because I'm looking at so many different ones, that's when the butterflies really start flapping. I actually had a great sense of what I wanted to happen in my book and tried to make it happen as plausibly as possible, and I did. Where it wasn't plausible, I made little changes. (Including changing Underwood's home state to Kentucky, the state of his birth, after a poll on this site.) I wanted an earlier Civil Rights movement, better race relations overall, etc.. And, it happened as plausibly as I could make it given the POD and help of baseball being integrated from the start.
However,w hen dealing with butterflies, if I don't know what I want to happen it can be a little overwhelming. I knew I wanted a Waldensian Reformation in "Sweet Lands of LIberty," and a few other things. The east/west splot of Catholic/Protestant Europe, the surviving Savoyard kingdom that grows a little and includes OTL Switzerland, and other thigns rose from that, and the world was made a slightly better place. But, if I'd gone too far in depth, I'd have never even gotten that short timeline done, because after that I wasn't really sure.
So, if this is going to be a novel, I'd do more than a timeline, but I still wouldn't take gobs of notes. However, if you have the time and the resources, go for it; just be careful not to get too bogged down.
I use mostly PDFs of books, and then use the search function in Adobe Reader to look for information I might find useful. But I just find reading shitloads of articles and documents on the area I'm researching gives me a better general idea than taking notes.
