Have you checked our book list located in the sidebar?

You may want to check out the online material related to the recent exhibit at the British Museum dealing with the Celts. This includes a nice online presentation - but there are actually several online sources associated with the exhibit, so you can Google "British Museum Celts" and you can explore some of the resources.

The thrust of the exhibit is the exploration of just what the term "Celt" means - the fact that in some ways the word has a history that transcends the history of any specific people. Various areas are and many more were linked linguistically, but what this implies by way of any ethnic or cultural homogeneity is another matter - and that is an academic battleground that's being tossed about on a regular basis without any clear consensus.

The exhibit - like modern-day scholarship - will do more to muddy the waters than to help you address "certain stereotypes" but if you explore this material, you will perhaps be able to put stereotypes into a certain perspective.

Edit: Here's another YouTube video on the exhibit. And se this lecture by Barry Cunliffe dealing with this subject some eight years ago.