This is the 956th most frequent Russian word.
собственных
"Own" or "personal" (possessive, plural or genitive case).
Here, "собственных" is used to denote something that specifically belongs to the individual.
Эти слова мои собственные мысли.
These words are my own thoughts.
Here, "собственных" emphasizes the personal ownership of beliefs.
Ты должен принимать решения, основываясь на собственных убеждениях.
You should make decisions based on your own beliefs.
Here, "собственных" is used in a possessive sense to highlight that the books belong to the individuals bringing them.