What is a habitual action in English?

You are right that one use of the present simple is to talk about habitual actions. In such cases there is often an an adverbial that explicitly indicates habitualness:

• I go for a run every day.
• She always brushes her teeth before breakfast.
• What do you usually do on Sundays? - I get up late and then go for a run.

Another use of the present simple is to state what could be described as facts without any implication of habitualness. Such facts include world truths like:

• Water boils at 100°C.

but also statements such as:

• He builds houses. She works as a manager. I go to college.

The above statements are akin to saying:

• He is a builder. She is a manager. I am a student.

The continuous aspect, by contrast, is used to focus less on the simple fact and more on the ongoing nature of the action. So in I am building a house and I am studying economics the emphasis is on the present and ongoing action.

The statement I read War and Peace can be interpreted as a habitual action or a simple fact. For example, in answer to the question: How do you send yourself to sleep?

But if you are talking about an action in progress, either at this moment or in the current period of time, then it needs to be:

• I'm reading War and Peace.

There is some slipperiness in concepts such as permanence, simple facts, and habitual/ongoing action. But they do help understand the basic usage differences between the present simple and present continuous.