Questions & Answers about Это очень интересная книга.
Why does the adjective интересная end in -ая?
It describes the noun книга (book). Because книга is a feminine noun, the adjective must match its gender. Most feminine adjectives in the nominative (subject) case end in -ая. The dictionary form (which is masculine) is интересный.
If книга is feminine, why do we start the sentence with the neuter word Это instead of Эта?
When you use a word to introduce something entirely new like "This is a...", you always use the fixed default word Это, no matter the noun's gender. You would only use the feminine Эта if "this" was directly attached to the noun to point a specific one out, like in the phrase эта книга (this book).
Where is the verb "is" in this sentence?
Russian generally skips the verb "to be" (am, is, are) in the present tense. You just put the subject and what it is side-by-side. So, the sentence literally reads as "This very interesting book", but it means "This is a very interesting book."