If you are interested in grading Sudokus by the difficulty of the human solving techniques required, you might want to check out the following two books:

- "Sudoku Programming with C" [https://www.apress.com/de/book/9781484209967] where a Sudoku solver and grader is implemented.

- "A to Z of Sudoku" [https://www.wiley.com/en-us/A+to+Z+of+Sudoku-p-9781847040008] where many human solving techniques are described and rated by their difficulty.