A guide is about a variable rather than a purchase — the thing upstream of which product you buy, and usually the thing that decides whether the purchase helps at all. No guide here ends in a shopping list.
It Was Never the Beans. It Was the Water.
There is a variable in your coffee that you never chose and cannot fix by shopping. It costs nothing to look up, it explains both the flat cup and the dead kettle, and almost everyone is wrong about which direction is better.