Federal Aviation Administration

Between this and the Instrument Flying Handbook, everything you need to pass the FAA's Instrument written exam. Could be usefully supplemented with, say, the Jeppesen book, which has many useful graphics, but a good basic resource.

Everything you need to know to pass the FAA's Fundamentals of Instruction test. There's a lot to get your head around if this is your first introduction to teaching, but it's reasonably well-presented and discussed. Definitely not the last word on instructing aviation, but also definitely a good introduction.

With the Instrument Flying Handbook, everything you need to pass FAA's Instrument written test. Could be usefully supplemented with, say, Jeppesen's Instrument/Commercial book (which has great graphics), but still a great resource.

This book contains all of the figures used in the FAA knowledge test for sport pilots, recreational pilots, remote pilots, and private pilots. I use it all the time when I teach ground school and give ground instruction.