You are right that the internet is full of nonsense on this topic. I would recommend you start with Barry Cunliffe, Iron Age communities in Britain : an account of England, Scotland and Wales from the seventh century BC until the Roman conquest 4th edition (London: Routledge, 2005). (link) This is a comprehensive academic source which has been updated with recent discoveries in the field. Like any good academic work, it utilizes citations and has a bibliography to send you to further readings on sub-topics (like particular sites). If you belong to an institution like a University, you can probably access this online for free. If you do not, send me a private message and I will help you get access.