I've enjoyed solving Sudoku puzzles for years, but lately I've been playing with other puzzles that are related to Sudoku, but are different. But I've recently begun reading books on solving Sudoku and I am back loving the basic Sudoku puzzle again.
The first book was HOW TO SOLVE SUDOKU: a Step by Step Guide by Robin J. Wilson. It was a free Kindle book when I got it from Amazon, but probably isn't anymore. I gave it 4 stars at Amazon because it did what it said it was going to do, but didn't go quite far enough. Most of what it taught I already knew, but I hadn't thought of doing what I was doing as a strategy and a process. The book is very clear and well written and if you are a beginning I suggest you get it.
The second book, which I am only about 1/3 through at this time is SUDOKU SECRETS by Amanda Ainsley. The writing in this book is not as clear as in Wilson's book, but if you work at it, you can learn what she has to teach. This is a much more advanced book. And that is its problem. Amanda Ainsley works too hard. She gets into the advanced problem solving without going through enough of a process to put in the easily solvable numerals. She doesn't seem to have a good starting strategy.
What I've discovered is that I can do all but the most advanced puzzles with a combination of all of Wilson's book, and just a hint of Ainsley's.
I start out by scanning the puzzle to see if I can easily figure out where a number ought to go. Frequently I'm not sure exactly, but can choose between two positions in a box using the information in the columns and rows. If I know that a number has to be in one of two or three positions in a line (either all in one column or all in one row) in a box I'll mark those boxes with small numbers.
My daughter once pointed out that sometimes people do crossword puzzles in ink, but everyone does Sudoku in pencil with an eraser handy. That is because the more difficult the puzzle the more you are going to be putting little numbers in a position on the grid. The little numbers allow you to see what the possibilities are for each position in a Sudoku puzzle.
The reason I say that Amanda Ainsley works too hard when she is solving puzzles is that it looks like the first thing she does is fill in all those little numbers. I generally have more than half the puzzle solved before I have to do that to get the rest of the puzzle done. And I rarely need to fill all of them in to finish a puzzle.
On the other hand, I'm unlikely to get much practice with the more advanced concepts if I keep finishing the puzzles before I try the advanced strategies.