One subject that is a necessary part of many areas of life, self sufficiency and survival is Knots.
Knots are used to tie things down, tie them up, hold things together and there are knots that need to slip, knots that must not slip and knots that must be quick release and those that should not. They are used to make bows and areas, tie fishing line, hold things together, secure bags, tie bandages, make a tourniquet, etc. there are uses in all walks of life. There are so many different types for so many different purposes that you really need to see them and apply them to learn.
Knots are another area that is covered in almost every survival book. Basic knot making is a skill that you need to have.
I have put some knot making books in the download area. Practise and if you can buy a paper version of your favourite to keep as a just in case.