The question presumes a stable employer/employee relationship.
For best economic security, ALSO focus on making yourself of maximum use to your surrounding neighbors.

how to make antibiotics, yes;
also how to extract proper dosages of salicilate [aspirin] from willow bark for fever reduction;
sorting out ineffective old wives tales from nativist pharmocopeia to give herbalism a scientific basis would be useful; you don't want to rely on myths when your patient could die if the diarrhea isn't stopped.
basic cpr, and other paramedic skills, even low tech battlefield condition medical knowledge would be handy.
midwifery.

but we are digressing from the letter writer's area of interest, engineering.
I suggest learning sustainable roman and egyptian and medeival technologies in addition to modern.

how to salvage useful materials and jeryy-rig them for functioning machines;
how to make and repair clocks, watches, and other automatic motion devices;
how to make a battery; repair solar or wind energy collectors
how to make a road that will last centuries
and low energy urban infrastructure redesign/planning: aquaducts and cisterns, nonflush sewage collection; permaculture,aquaculture, food storage science, etc