This is the 7787th most frequent Russian word.
интересная
"Interesting."
'интересная' is used as an adjective to describe the noun 'книга' (book), feminine singular nominative.
Эта книга действительно интересная.
This book is really interesting.
'интересного' is the masculine singular accusative form of the adjective, matching 'человека' (person).
Мы встретили интересного человека вчера.
We met an interesting person yesterday.
'интересную' is the feminine singular accusative form of the adjective, agreeing with 'историю' (story).