How much wider reading should I do?

Textbooks and a few core texts recommended by your teacher will often suffice, however reading a few good overviews of the period will help you to better understand it and provide examples or explanations not mentioned in class. However it is better to know the course content better than to be distracted by detailed academic arguments. For coursework, wider reading is necessary however it will tend to be more specialised and will probably be limited to twenty articles or chapters of books.