I have come to flight testing late in my career (just received my first flying FTE authorisation at the age of 51) and found both Kimberlin and Stinton very helpful indeed when learning about it a couple of years ago. I was very lucky to get the Stinton one for fifty pounds last year at an Old Warden event. Internet prices are much higher. If you are new to flight testing, John Farley's A View from the Hover is not a text book but does, especially in the second half, provide a very useable and entertaining introduction to the ethos of flight testing.
A previous employer put me through a very focussed one-to-one introductory course to be a ground based FTE. My tutor for that training (if he's reading this, it was a job well done) recommended the following publically available materials which I found to be useful:
- The FlightTest Guide part of EASA CS-23.
- FAAOrder 4040.26B flight test risk management program
- Cooper-Harper report NASA TN D-5153
- AGARD Introduction to Flight Test Engineering
You will need to decide for yourself which bits of the large FTG and AGARD documents you need to study.
This EASA presentation is useful as well:
https://www.easa.europa.eu/system/fi...ganisation.pdf