Can you explain a little bit further, such as your preferred period and whether you have access to a good library? My research on homage has been focused on medieval Wales between c.1090-1283 (the Edwardian Conquest) so I can recommend books on a range of topics and at varying levels of interest (ie. pop history to quite very complex primary sources). My period might be a bit outside of what most people think of when they talk about 'Celts' (especially as the Welsh were not calling themselves such).

The best and most comprehensive new book on early native society in Britain is T.M. Charles-Edwards, Wales and the Britons, 350-1064, (Oxford, 2013) but its rather pricey (£100!). If you are interested in the period after this then almost anything by Rees Davies will do, his prose is incredibly accessible and he was the foremost Welsh scholar until his recent death.