This is the 228th most frequent Ukrainian word.
ця
"This" (feminine singular).
Here, 'ця' is a demonstrative pronoun modifying 'книга' (book) and means 'this' in the nominative case.
Ця книга дуже цікава.
This book is very interesting.
In this sentence, 'цю' is the accusative case of 'ця', used because 'книгу' (book) is the direct object.
Я взяв цю книгу на урок.
I took this book to the lesson.
Here, 'цю' is in the accusative case used with the preposition 'про' and modifies 'книгу' (book).