Sudoku databases
Free database with 3 million puzzles (csv format): https://www.kaggle.com/datasets/radcliffe/3-million-sudoku-puzzles-with-ratings
History of Sudoku
A talk given in 2007 by Peter J. Cameron (pdf format), in which I discovered the concept of Gerechte Designs: https://webspace.maths.qmul.ac.uk/p.j.cameron/talks/sudoku3.pdf
Looking for more information about Gerechte Designs, I found the following paper entitled Sudoku, Gerechte Designs, Resolutions, Affine Space, Spreads, Reguli, and Hamming Codes: https://doi.org/10.1080/00029890.2008.11920542