Julian Eastman is only mentioned a couple of times in the book. I believe the first time his name comes up is in Chapter 6, when the young lawyer, Edgar Jennings Plum, reads the will of Samuel Westing to the game players. He notes that the signature on the will is Samuel Westing's, and that his two witnesses are "Julian R. Eastman, President and Chief Executive Officer of Westing Paper Products Corporation, and Sydney Sikes, M.D., Coroner of Westing County".
Julian Eastman is mentioned again in Chapter 18, when Turtle reads from The Wall Street Journal that he was "the newly elected chairman of the board of Westing Paper Products Corporation". Mr. Eastman would be "conferring with European management that earnings from all divisions (were) expected to double in the next quarter".
Julian Eastman does not appear in person in the book (or perhaps I should say that Windy Windkloppel does not appear as Julian Eastman in person in the book) until Chapter 27, when Turtle figures out the last piece of the puzzle. She already knows that Windkloppel has used three aliases, each of which represent one of the four directions on the compass - Sam Westing, Barney Northrup, and Sandy McSouthers - and there is one more identity to find. The fourth identity is, logically, Julian Eastman, and when Turtle finally discerns this, she goes to "the modern mansion of the newly-elected chairman of the board of Westing Paper Products Corporation" and requests an audience with Mr. Eastman, who has been expecting her.