I overheard this described but did not get a title and now I really want to buy it. In this book the characters suddenly realize they can make whoever is reading it say whatever they want. Later, the characters become worried that the book is going to end. What is the title?
We Are In A Book by Mo Willems may be it. It’s pretty much as you described and my son loves it.
Along similar lines, the Sesame Street book There’s A Monster At The End Of This Book has Grover worrying about the monster at, well, the end of the book, trying to get the reader to not finish reading. Then he realizes that he was the monster all along.
There’s also a sequel – There’s Another Monster at the End of This Book. It stars Grover and Elmo.
I got about halfway through the OP and started reading it in a Grover voice.
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Not what the OP is thinking of, but has anyone read Sophie’s World?
that was my first thought as wel, well that and the last part of “his dark materials”
of and bits of the neverending story