Lambert's study is both comprehensive and enthralling. Definitely a must for any undergrad/postgrad course on medieval heresy. However, the direct link which Lambert draws between Cathars and Bogomils is an interesting but questionable thesis - it should not be represented as fact. Nevertheless, I have a lot of love for this work.
As a history sudentent Christopher Hill appeals to me because of the way he presents his ideas. His works are great in helping you develop your own ideas, partly because of his selective use of evidence, he gives you something to contest. Christopher Hill has an agenda - but who doesn't. The same can be said for any text- you must read with care. He has a fantastic style and this work is one of my favourites. 'The World Turned Upside Down' is engaging and exciting.