FAA's Flight Instructor manual. Free download from the FAA's website. Covers theory of instruction etc quite well.
https://www.faa.gov/regulations_poli...tors_handbook/
There are also lots of other reference manuals/handbooks on their site, but bear in mind the FAA orientation of it all.
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NZ CAA's instructor briefings. Various briefings & diagrams. Free download. Powerpoint presentations for a stack of briefings, diagrams etc
https://www.caa.govt.nz/FIG/index.html
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Oz, CASA's 'Flight Instructor Handbook', formerly known as 'Publication 45'. Quite good for covering what to teach in the air, what to look for etc.
https://www.casa.gov.au/standard-pag...nual-aeroplane