Although Karl Barth wrote many books, in order to fully engage Barth, the Church Dogmatics must be read first. Most people only engage Barth's Evangelical Theology, or possibly his commentary on Romans (Epistle to the Romans), but Barth wrote in the preface to the sixth edition of the commentary on Romans that he now realized that he'd have to rewrite the Romans commentary and revising it would not be possible (paraphrase). Barth originally titled his Dogmatics, "Christian Dogmatics", but abandoned this as a false-start before completing it. And restarted at the beginning in his Church Dogmatics. I've read part of the first half volume of the Dogmatics called "The Word of God Vol. 1.1", and recently skipped ahead to Barth's "Doctrine of God" (Vol 2.1 and 2.2).
Karl Barth's Doctrine of God are printed in two books: Volume 2.1 and Volume 2.2 of his Church Dogmatics. I'm reading the older T&T Clark editions of the Church Dogmatics. Unfortunately, there are long sections of untranslated latin and greek quotations that make Barth's arguments difficult to follow without a translation. Barth writes a summary of what he believes in large print and immediately follows that summary with a long paragraph with the "fine print" full of latin and greek quotations from church fathers that justify what Barth had wrote in the summary. However, if you want to spend the money on the Church Dogmatic 31 Volume Study Edition, it has the translations inline. I am not made of money, so I was only able to purchase a used copy of The Doctrine of God (Volume 2.1 and 2.2). I've recently completed reading Volume 2.1, which is almost 700pages, but half the pages are in small print, so it reads more like a 1,000 page book. Now, I'm a hundred pages into Volume 2.2, and this second part is a hundred pages longer than the first volume.
Unlike most Systematic Theologies, Barth's Dogmatic places The Doctrine of God in Volume II, following his two-part Volume I on the Doctrine of the Word of God; which was exemplary of Barth's Christocentricism. I may talk about Barth's understanding of the Word of God another time. The Doctrine of God unfolds the ground work of the introductory Volume's 1.1 and 1.2, and begins the fullness of Barthianism that is unpacked in the rest of the 13 volumes.
Karl Barth is a Swiss Reformed Theologian, who wrote during and after World War II. He has been increasingly more famous since the Dogmatics were published. His writings directly engaged great theologians, and he is especially famous for his criticisms of Schleiermacher's "The Christian Faith", which I read last year. However, there are many infamous critics of Barth deep in the heart of the American Reformed Community. A notable example are the followers of Cornelius Van Til, as well as Van Til's book, "Christianity and Barthianism", which may be the worst published book on Karl Barth of all time.
Related: Church Dogmatics, Doctrine of God, Karl Barth, Vol 2.1, Vol 2.2