Past Master is an idiosyncratic book, written in a strange style, which takes a while to acclimatise oneself to. But once up to speed with Rafferty's language, the story is difficult to put down. I recommend setting aside a couple or more uninterrupted hours to best savour the flavour!
Thomas More is brought forward in time to solve a political problem on a supposedly utopian world, but the puzzle is not as simple as that. In some ways his presence is merely an excuse for everything else that goes on between a diverse cast of human and alien beings, all beautifully crafted and who leave a lasting impression in our mind. But then their actions are even more peculiar, and we need to travel right to the end of the book, through all of the book, to be able to comprehend what is going on. Skimmers beware, it really does make sense, but only if sufficient time has been allowed to read it properly.
Underlying the main theme of high adventure is a foundation of wit in various forms, sardonic, raucous, tender, even tragic, depending on the circumstance and individuals. The flavour of the narrative adds to the atmosphere, and once inside the story I would not want it any other way.
This is a book every SF enthusiast should have read, perhaps even owned and treasured. I bought my Ace copy, as shown here, when it came out in 1968, and have just enjoyed reading it again. I forget how many times I have re-read it in the intervening years.