Dozens of Go books are available as in-app purchases, including the classic Invincible, by John Power, which looks at the games of Shusaku, and which is considered to be the best book on the game of Go ever published in English. (Disclaimer: I was one of the reviewers of the Go Books edition of this book.) But you can also buy Janice Kim's Learn to Play Go series, the Graded Go Problems for Beginners series, and a number of Workshop Lectures by Yang Yilun. With the right amount of time, and the books available in this app, you could learn almost everything about the game.
And if you want to play against others around the world, check out PandaNet (free), a Go server app. Connect to the server and play with people of all skill levels, and even watch pro games and tournaments that are broadcast on the server.
You've got a lot to choose from, but if you really want to study the game, SmartGo Kifu is the obvious choice. If you're a beginner, either Igowin Pro or SmartGo Player will get you started, and if you want to learn more, the Go Books app will help you build your own library of great Go books on your iPad.
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