Several times in my life, when I've needed to study/learn something, I just found or made a wiki on it, and categorized everything which I learned on it --- then when I had trouble recalling a fact, it was there in that structured site --- on the flip side, there almost certainly are folks who will aver that I ruined the Shapeoko CNC project wiki by using it as a personal notebook. Fortunately, @julien, a native French speaker from the Carbide 3D Community forums wanted to improve his English, so he made a gitbook:

https://shapeokoenthusiasts.gitbook.io/shapeoko-cnc-a-to-z/

which re-worked the essentials from that wiki, discarded the chaff, and has become a reference which a number of projects have re-purposed. I did resurrect the notes aspect on the /r/shapeoko wiki though.

Similarly, when I wanted to set up the ultimate commuter/long-haul mountain bike, I put down all the gear I learned about at:

http://old.reddit.com/r/bicyclegear/wiki

(probably out-of-date now, but I found the notes useful)

Unfortunately, I've lost access to the two e-mail archives from when I worked as a graphic designer/typographer --- really should have forwarded any notable e-mails (which I would have wanted to refer to later) to myself --- at least one of them wound up being printed out by a startup composition house and distributed to new employees.... maybe one of these days I'll finish the type composition book I was asked to write by an editor at a major publishing house.

For now, I've been working on:

https://willadams.gitbook.io/design-into-3d and https://github.com/WillAdams/gcodepreview

How to record and retrieve anything you've ever had to look up twice