Learn Russian: Я читал эту книгу дважды.

In Russian, when an action happens more than once in the past (like reading a book дважды - twice), we generally use the imperfective aspect. Even though you finished the book both times, the repetition makes it a recurring event, which triggers the imperfective читал.

Since we are learning about habitual verbs like бывать, could I use бывать here to show I read it habitually?

No. Unlike English, which sometimes uses 'used to' as a helper verb, бывать is a standalone verb meaning 'to visit regularly' or 'to tend to be'. You cannot pair it with читал. However, this sentence fits the lesson's theme because it shows another way Russian handles repeated events: simply by using an imperfective verb with a frequency word like дважды!

Is there a difference between saying дважды and два раза?

No, they mean exactly the same thing: 'twice' or 'two times'. Дважды is a single adverb, while два раза uses the number two plus the genitive singular of раз (time or occurrence). You will hear native speakers use both interchangeably.

Why is it эту книгу instead of эта книга?

Because 'this book' is the direct object of the verb читал (it is the thing being read), so it must be in the accusative case. The feminine nominative phrase эта книга changes to the accusative эту книгу.