Learn Hardcore Russian: Он внимательно читает книгу.

Russian simply doesn’t have articles.
There is no separate word for a/an or the, so книгу can mean a book or the book, depending on context.
If you really need to show that the book is specific, you usually add other words (like эту книгу – this book).

Why is it книгу and not книга?

Книгу is the accusative singular form of книга.
In this sentence, книгу is the direct object of the verb читает (reads), and in Russian direct objects normally take the accusative case.

Declension of книга (singular, main cases):

Nominative (subject): книга – The book is on the table.
Accusative (direct object): книгу – He reads the book.

So Он читает книгу = He reads (is reading) the book.

What form and tense is читает, and how is читать conjugated?