How do express the difference between "I read books" and "I am reading a book"? Does it just depend on context?
Just like that,
Я читаю книгу. — for a book you are reading at the moment (accusative of singular книга),
Я читаю книги. — for books in general (accusative for plural is the same).
But instead of the second one in a conversation about your hobbies I would rather say 'Я много читаю.'