bookends, book ends

Bookends or bookends ( one word ) means a pair of heavy objects used to keep books standing up on a shelf, desk, or table--
He has two old bronze gargoyles on his desk, for bookends.
Two bricks make useful, utilitarian bookends.

Book ends or book ends ( two words ) is the third person singular ( he, she, it ) form of "end", meaning to come to a conclusion or to  finish, said of  a literary work of some kind ( book )--
Don't tell me how this book ends! You'll spoil it!
I can't wait to find out how this book ends!

Now that you know that, you can say--
"The book ends when the detective discovers the body of a man murdered with one of his own bookends."

Note--"books end" means the finish or conclusion ( end ) of more than one literary work ( books)--
Many of his books end with a surprise.
Note # 2--"book's end" means the finish or conclusion ( end ) belonging to one particular literary work ( book's )--
This book's end is a real shocker!
Note # 3--"books' ends" means the conclusions ( ends ) belonging to more than one literary work ( books' )--
This authors books' ends are always exciting. [ awkward but logical ]