You can go through a lot of end mills cutting dry. This is less of an issue for hobbyists who aren't turning large volumes of metal into chips and aren't using high-powered milling machines. The main limit on milling speed is getting rid of the heat. If you're willing to run slow, dry cutting works. Or if you only cut materials softer than steel.

At some point, you get Machinery's Handbook.[1] For most of a century, machinists' toolboxes had a built-in space for a copy of that book. Now it's available as an app.

[1] https://books.industrialpress.com/machinery-handbook/