This is the 11597th most frequent Russian word.
единственном
"Единственном" means "the only" or "the sole" in English.
Here, "единственном" is used to mean "single" or "unique" to indicate there was only one instance of the book.
В единственном экземпляре эта книга была передана в библиотеку.
This book was delivered to the library as a copy of its kind.
In this sentence, "единственном" describes a person being the sole individual capable of doing something.
Он был единственном в своём классе, кто смог решить задачу.
He was the only one in his class who managed to solve the problem.
"Единственном" is used here to indicate exclusivity in terms of the friend who remained supportive.