Learn Russian: Это книга.

When you use это to introduce or point something out (meaning "this is" or "it is"), it acts as a fixed introducing word. It stays exactly the same, no matter the gender of the noun that follows.

Where is the verb "is" in this sentence?

Russian usually drops the verb "to be" in the present tense. Instead of saying "This is a book", you simply say "This a book."

There is no word for "a" here. How do we know it's "a book" and not "the book"?

Russian doesn't have articles (the words "a", "an", or "the"). Depending on the context, книга can mean either "a book" or "the book". When introducing something new with это, it usually translates best to "a book" in English.

How is the 'o' at the end of это pronounced?

Since the stress in это is on the first letter (Э), the о at the end is unstressed. In Russian, an unstressed о sounds like a short, relaxed "a" (like the "a" in "sofa").