I found this question difficult to put into words. I'm reading 'A Brief History of the Celts' and I'm enjoying it for the most part. This is the first time I've looked into history outside of school for my own self interest, so forgive me if I seem vague about what I'm looking for.
I would like to know more about Iron Age Britain, the culture of the Celts and other peoples, and their confrontations and partnerships with Rome. I've seen such films as King Arthur (2004), The Eagle (2011) and Centurion (2010) and, I'll admit, I'm a sucker for the romance of old Britain and the conflicts which took place.
I prefer to learn through personal accounts rather than documentaries or historical books (though I'm very open to suggestions for those). It's clear there in fiction at least the Celts (god I feel like I'm using this word completely wrong) are viewed as savages by the Romans who, as the victors, wrote the history books and subsequently became the heroes of Hollywood's movies.
Unfortunately, Iron Age Britain is a pre-literate culture so you won't be able to find any "personal accounts", especially not from an actual Celto-British perspective.
What we know about this period and these peoples thus comes from archaeological records, not literary sources.
Roman sources start to take an interest in the Celts and specifically the British tribes round about the turn of the first millennium, as the Roman Empire (or pre-Empire) started to actively push its way west. Of course the problem with these sources is that they are written from an outsider's perspective and what ethnography they engage in is heavily biased and shaped by the authors' perspective and rhetorical goals. They are worth reading, but they must be read for what they are, accounts by and for Roman elites not in depth analysis of other cultures.
Julius Caesar's Gallic Wars provides info on Celtic Society (primarily that of Gaul) and a little bit on England.
Tacitus' Agricola provides some information on the Britons, but it is not a particularly "accurate" account, given that he is writing several decades after the initial Roman entry into England and is not himself a primary observer. The work is also heavily shaped by his own Roman mindset and particular rhetorical goals. It is also worth remembering that Tacitus is writing several decades into Roman occupation of Britain.
An expert on Ancient history may be able to give you some more to go on (espeically secondary sources), and you might also want to swing over to r/AskAnthropology and see if you can track down an archaeologist there.