This is a exceptionally interesting book. You'd need to have read a fair amount of Twain to get much out of it because it's a quick series of references to his works and his life for which you already need context. But if you're familiar with Twain, this book has a lot of insights about what he was doing and how he changed literature and American culture.
The author is (was) a professor, and I assume the chapters and sections are pieces of lectures. They don't fit together very well sometimes, but he keeps returning to a few key themes, and that's how a picture emerges. That picture is of a writer who always had humor at the core of what he did but, at the same time, felt embarrassed that maybe humor wasn't the highest calling of literature. However, if someone criticized him for being "merely" a humorist or comic writer, he'd fight back with satire.
The book points out many things that weren't obvious to me. For example, he wrote 7 novels (I think that's right) and nearly as many travel books, and it's why he is most known today. But in his day, he was better known for his short, topical pieces that appeared in leading magazines and newspapers constantly. He was always called upon for an opinion or a quip on a current topic, and he usually obliged. He was like Steven Colbert or some other late-night comic, but with everyone paying attention to him. Most of the novels and travel books were bestsellers in their day, but some didn't get great reviews. They were not esteemed, and in fact, most were sold on a subscription basis by house-to-house peddlers, rather than preprinted and available in bookstores. Bookstores were for quality, serious literature.
As described by Ziff, a lot of Twain's work involved a metaphysical journey that paralleled the physical journeys so common in his books. These were young men, innocents abroad so to speak, who went somewhere and did some stuff and emerged more knowledgeable, more worldly, and also more humble about their place in the world. Twain liked to pretend this was his path as well, but it wasn't quite. For example, in writing about both his two weeks with an informal rebel militia at the start of the Civil War and then his time as a prospector in post-Gold Rush California, he portrays himself as a waif, a kid barely out of knickers, who was amazed by everything he saw. Actually, he was 26 when he did his militia stint and then escaped to the West, and he'd already been trained in the extraordinarily hard job of being a Mississippi River pilot. He only stopped doing that job because shipping the river was ruined by the war's hostilities.
Twain was a keen observer of human folly -- that's pretty obvious. This book does a great job of giving examples of it that are hugely entertaining and go beyond the ones from his well-known works. At the same time, there was an edge of defensiveness to Twain about these things that isn't always appreciated. In particular, his first popular book, "Innocents Abroad," shows him as the ignorant American taking a round-the-world trip with other ignorant Americans, reveling in the fact that he doesn't like Renaissance paintings or fancy churches. To a large degree, this was true. He didn't have a ton of education, and he didn't understand the great works he was seeing, and he was ok with that. He turned travel writing on its head by saying that it's ok to not understand something or even not like it. You don't have to listen to the alleged experts (in this way, he was a century-and-a-half ahead of the ignorant Republicans today who doubt vaccine developers, climate change scientists, and school principals). But the book also explains how Twain matured. Late in life he wrote another travel book, and this time he focused on the savagery and inequity he saw in repressive, genocidal regimes around the world. He wasn't an "innocent" abroad any more; he was a thoughtful person who did what he could to right the world's wrongs.
There are many long biographies about Mark Twain, and it's worth reading one or two of them. This book is not a substitute, by any stretch. But it hits some of the high points of his literature, and it puts it in context of the America in which he was living --- a time of great economic and social change, but also reactionary forces that stunted the lives of many, especially Black Americans. Twain saw it all, commented on it all, and was generally right about it.