Formal:
* At the conclusion of the book: This is a very straightforward and formal way to say it.
* In the book's final pages: This emphasizes the physical aspect of the book.
* Upon the book's completion: This highlights the act of finishing the book.
Informal:
* By the book's end: This is a more conversational way to say it.
* Towards the end of the book: This is a more subtle way to say it.
* As the book winds down: This suggests a gradual approach to the ending.
* When the book wraps up: This is a casual and playful way to say it.
Literary:
* In the book's denouement: This refers to the resolution of the plot.
* At the book's climax: This refers to the most exciting or important part of the story.
* In the book's epilogue: This refers to a section that comes after the main story.
The best choice for you will depend on the specific context and the tone you want to convey.