commonjong VS RiichiBooks

I've been working for quite a while on the back-end of that client, the link to my code repository is https://github.com/bonarc712/commonjong. If you would like to use some parts of it, you can go ahead!

As for learning, there's Riichi Book I by Daina Chiba here, which I think focuses on EMA Riichi Mahjong, so uses the european terminology borrowed from Chinese mahjong (so pung=pon, chow=chi, kong=kon, etc), but is otherwise the same game. And the Riichi Wiki here.

The best place to start imo is to brush up on fundamentals. Riichi book 1, found free here: https://dainachiba.github.io/RiichiBooks/ has a lot of good fundamentals to look out for. Focus on 5 block theory, defense, and tile efficiency if it's too overwhelming to start!

This might be overkill, but I learned a lot from this: https://dainachiba.github.io/RiichiBooks/ Basically a Riichi mahjong textbook. Section 8.2 sounds like what you're looking for at the moment.

Another important thing is to have better tile efficiency. For a good beginner/ intermediate introduction to defence and tile efficiency, read Riichi Book 1, or as the community calls it, the Riichi Mahjong Bible

The often recommended Riichi Book 1 has a section on reading and defending against open hands that is worth looking at.

Here's Riichi Book 1: https://dainachiba.github.io/RiichiBooks/

Chapter 8 of Riichi Book 1 (available for free here) does a great job of explaining the fundamentals of defence. Then to directly practice you can use Erzzy's folding trainer and it corresponds directly to the levels of safety described in riichi book 1 (this is a link to her efficiency trainer, but a folding trainer is available within the same thing once you run it). For people that don't speak Japanese, riichi book 1 is the go to answer for learning any of the fundamentals of the game :) .

You can also take a look at the defense judgement section of riichi book 1.