Most good ideas (whether they’re ideas for narrative structure, a particular twist in the argument, or a broader topic) come into our minds as hunches: small fragments of a larger idea, hints and intimations. Many of these ideas sit around for months or years before they coalesce into something useful.
Good ideas collect like pennies and come together when you least expect it. However, unlike the author I feel like your best notebook is the one you have with you.
My fastest notebook is my Smartphone. I type on a touch screen much faster than writing brain farts down with a pen where I’m more likely to lose an idea.
Do whatever works for you but try to pick one source so you can easily come back to it and build upon your ideas. Some ideas are so random and need to soak.