In the book, the man in the high castle is the author of the book within the book, The Grasshopper Lies Heavy. In the television series, the man in the high castle is not explicitly named, but is presumably Hitler. He's shown living or at work in a high castle, with newsreels of the alternate histories on his shelf. The man in the high castle was earlier identified as someone who collects the newsreels - that's why the resistance and the Nazis want them. But this still leaves open the central question of: who created in the newsreels in the television series? As one of the characters in the resistance remarks, why would the person who is collecting the newsreels be the same person who created them? Perhaps Hitler has some as yet unfathomable agenda - which, at this point, as the first season ends, is just to know the alternate histories that the newsreels are revealing.
The difference between a book within a book in the novel and newsreels in the television series as media of alternate history lead to another difference between the Philip K. Dick novel and the Frank Spotnitz television series that in many ways is the most profound difference of all: When Juliana and Frank see Joe as a Nazi killing Frank in the newsreel at the end of the 9th episode, there's no doubt that the people in the newsreel are indeed Frank and Joe. The characters can't make much sense of this, but we the viewers understand that we're literally seeing either an alternate history or the future. This literal look at an alternate history is much more powerful, metaphysically, than anything that can be described in a secret book. Writing, is, after all, a description of people and events. In contrast, visual media such as newsreels are literal recordings of those people and events. Of course they can be distorted and manipulated, but at their very basis, literal images traffic with the truth in a way that written words cannot. (See any of my books about media theory for more.)
Newsreels were of course around when Dick published the novel in 1962, but he chose, for whatever reason, to go with the book within the book as the alternate history medium (whether he didn't think of newsreels, or rejected them as a vehicle for some reason, we'll never know). But brilliant of Spotnitz to make them the vehicle of the television series - starting with the newsreel at the beginning of every episode - and open up so many new and compelling possibilities.
What if the Soviet Union had survived into the 21st century
more time travel and alternate history