As the book suggests, the correct form is 'en casa'. You could also say: 'Mi amigo está en SU casa'. This one is less common but still correct. Some Latin American people (I don't want to include everybody) tend to use 'Pedro está en LA casa'. I am not sure whether this is correct or not over there; however, I am 100% sure that is not correct in the Spanish spoken in Spain.

If you use "en casa" it means at home, and i thing that is what you want to say. But sometimes you want to be specific, and for some reason you want to use 'en la casa'. For instance, you are travelling and you rent a house with some friends, and someon ask you where somebody is, so in this case you would say 'mi amigo esta en la casa' and not 'mi amigo esta en casa', because 'mi amigo esta en casa' means at home .... at his home, and in this particulary example he is in the rented house, so it would be correct to say 'mi amigo esta en la casa' (my friend is in the house)

If there is no more specification, in Spain, in general, we say 'en casa'. We understand it is the usual home. We use the article only if we refer to a specific house (other than the usual home). For example: 'en la casa del pueblo'. In other countries, they say always 'en la casa'. So you will hear 'en casa' or 'en la casa' depending on the country of origin of the people you talk with. It is more a matter of use than a matter of correction.

I am not sure but I read it always en la casa

To me the correct one is "mi amigo esta EN casa".