I've found the best machining resource is working with a good machinist. Quite a bit of machining is about intuition and properly evaluating other sensory inputs - knowing how to interpret the sound a cutting tool makes is critical! Unfortunately, finding a good machinist willing to let you watch them in person - or better yet, work under their supervision - is pretty hard, especially if you're just a hobbyist. Crotchety old-timers don't scale particularly well...

Youtube is one of the better resources right now, for anything other than really basic calculations. There seems to be a trend of old-school professional machinists documenting jobs they're working on - sort of a cross between a tutorial and virtual job-shadowing. Some nice users include oxtoolco, KEF791, Abom79, and mrpete222.