Play a lot
With what you've read so far, you have everything you need to play a full game and follow its rules. But mahjong is a deep game, and learning it properly takes years. The only way down that road is to play.
Play wherever you can: at the nearest club, at home with your family, or online on Tenhou or Mahjong Soul, against players from all over the world. If you want someone to look at your games, I review game logs and coach players one on one. The services page has the details. The free stats tools on npmahjong.com are built on real game data and answer questions like how often your wait wins and when to push against a riichi. Every time you sit down at a new table, ask about its local rules: the rules and penalties chapter tells you which ones tend to change.
Ask, and read it again
Stay curious. Ask about anything you don't fully understand, to other players or to me, through npmahjong.com. Read this book, then read it again: the second time you'll catch things that slipped past you the first. And keep looking for new books and material.
Find your own way to play
There are many ways to play. You can play occult (オカルト, okaruto), trusting streaks and gut feeling, or digital (デジタル, dejitaru), trusting probabilities and data. You can play it safe and defend, or attack every hand. You can aim to be a strong online player or a recognized pro. Your style doesn't matter as long as you're having fun, you stay interested, and you want to learn a little more every day.
In the end, mahjong means sharing a table with three other people who are having a good time and understand each other through the thing they have in common: this game. Carry your place as a mahjong player with pride and with responsibility. Mahjong is young as games go, and few are as addictive.
Welcome to mahjong.