[charlie]

I have compiled a short list i had already im my files:

at first: Check the internet for Presentation,Lectures, Papers of Uwe Ewert and Uwe Zscherpel from BAM, already here in NDT.net you find a lot. also check Researchgate.net (https://www.researchgate.net/profile/Uwe_Ewert2 , https://www.researchgate.net/profile/Uwe_Zscherpel ) . Even videos can be found – e.g.: https://www.youtube.com/watch?v=xefUp80WSfc
BAM is a leading Institute in this field as they work also as a leading body in standardisation e.g. they do contribute to the ASTM Standards – see here
https://www.astm.org/search/fullsite-search.html?query=digital%20radiography&

one book from ASTN regarding DR – an older version is linked here (legal?)
ASNT_Level_III_Study_Guide_Radiographic – you get from ACADEMIA (https://www.academia.edu/38830479/ASNT_Level_III_Study_Guide_Radiographic_Testing_Method)
also this book:
Handbook of Nondestructive Evaluation -Charles J.Hellier – you get from ACADEMIA https://www.academia.edu/35003120/HANDBOOF_OF_NONDESTRUCTIVE_EVALUATION

The IAEA has a lot of valuable documents:
IAEA( https://www.iaea.org/publications ) – check this page about radiography publications, there are some publications nemed
https://www.iaea.org/publications/6382/training-guidelines-in-non-destructive-testing-techniques-2002-edition (Training Guidelines in Non-destructive Testing Techniques 2002 Edition)
https://www.iaea.org/publications/8055/training-guidelines-in-non-destructive-testing-techniques (Training Guidelines in Non-destructive Testing Techniques, 2008 Edition)
https://www.iaea.org/publications/10765/training-guidelines-in-non-destructive-testing-techniques (Training Guidelines in Non-destructive Testing Techniques, 2013 Edition)
https://www.iaea.org/publications/10915/training-guidelines-in-non-destructive-testing-techniques (ultrasonic)
https://www.iaea.org/publications/10833/guidelines-on-training-examination-and-certification-in-digital-industrial-radiology-testing-rt-d (Guidelines on Training, Examination and Certification in Digital Industrial Radiology Testing (RT-D), 2015)
https://www-pub.iaea.org/MTCD/Publications/PDF/TCS-60_web.pdf
https://www.iaea.org/publications/8614/training-guidelines-in-non-destructive-testing-techniques-manual-for-visual-testing-at-level-2
https://www.iaea.org/publications/8696/training-guidelines-in-non-destructive-testing-techniques-leak-testing-at-level-2
https://www.iaea.org/publications/8414/eddy-current-testing-at-level-2-manual-for-the-syllabi-contained-in-iaea-tecdoc-628rev-2-training-guidelines-for-non-destructive-testing-techniques
https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1561_web.pdf
IAEA TECDOC No. 628, Training Course Series No. 60, No. 67, No. 54, No. 52, No. 48, No. 3 are the NDT Training courses
In canada I found this some times ago:
Examination Guide forInitial Certification, Industrial Radiography, Aerospace Sector http://publications.gc.ca/collections/collection_2018/rncan-nrcan/M39-140-2002-eng.pdf
http://publications.gc.ca/collections/collection_2017/rncan-nrcan/M39-136-2002-eng.pdf

Some more links to ASNT documents
https://automaterials.files.wordpress.com/2018/09/radiographic-testing-asnt-3rd-ed.pdf
http://www.gbv.de/dms/tib-ub-hannover/716053276.pdf
https://www.bakerhughesds.com/sites/g/files/cozyhq596/files/acquiadam_assets/industrial_radiography_image_forming_techniques_english_4.pdf

This is also an important source:
http://www.dwgndt.org/Digital%20and%20Film%20Compare%20%20and%20Contrast%20Handout.pdf
http://www.dwgndt.org/fwgidrfiles.htm