One thing with books that most modern entertainment lacks is that you know when it will end. You can tell as you go through a book how much remains. This reduces the uncertainty present in the plot.
You could add empty pages to the end of the book. But these would be obvious. Adding complete gibberish would also be obvious even to someone flicking to find the last page number. You could generate more realistic text using a Markov model. Shannon generated fairly reasonable looking text with a simple algorithm. Markov models will create text that bears closer inspection though. It was used to generate postmodern critical analysis that got published without anyone realising it was nonsense.
So you train up your model on the rest of the book. Then get it to produce pages of text that without some study cannot be differentiated from the real books text. Then place these pages with the occasional *the end* in them after the book ending. The ending is marked with *the end*. Now the readers who do not go to a big effort cannot tell as they read the book how much remains.