There’s loads of good books like Machinery Handbook but I generally just keep a set of Roebuck Zeus Tables in my pocket.
I love the Workshop Practice Series books including this one on the screw cutting lathe[1] is a good introduction.
As much as possible I stick to the ISO Metric Course thread standard. That’s the M sizes.
I can get away with just one set of change gears, taps and dies and one set of tapping drills.
E.g. to tap an M8 size hole you need a 6.8mm drill so that when you tap it it will have the correct size. You generally won’t find that a set of drills from a DIY store.
I avoid it but if I do need Metric Fine threads I can cut them on the lathe with the same set of change gears.
For other non-metric standards for a specific purpose I buy one off taps and drills but I really try to avoid this.
e.g. camera mounts are usually either Whitworth or UNC but you won't find these much else where.
Quality drills and taps really help with small lathes where motor power is limited. I’ve tried cheap stuff but it's somewhat frustrating when a cheap, no name tap breaks off in something you've spent days making. Now I just use Presto[2][3] for everything.
