I recommend Philip Freeman's books for rigorous scholarship bearing fruit as an entertaining and interesting book. Freeman is the author of St. Patrick of Ireland: A Biography, as well as The Galatian Language (he does his own translations of the material from Greek, Latin, and various ancient Celtic languages), and several other books drawing on the latest philological and archaeological evidence to cast light on the Gauls and Celts of antiquity.
The present book, The Philosopher and the Druids: A Journey Among the Ancient Celts (2006) is focused on the Gaullic tribes in the first century before Christ, a time of expansion of the Roman Empire and increased contact for the two cultures. An Athenian-educated philosopher named Posidonius undertook a trip deep into tribal territory, recording some of the first known ethnological and cultural notes on the Celts in classical sources. You get the sense that a host who has his enemies' heads hung on the dining room wall might actually be the kind of guy you'd like to have a beer with. The equally tough Celtic women are rightly emphasized, with some discussion of their cultish temple communities.
Freeman is scrupulously careful about sticking to the known facts which pays off with a sense of satisfaction that one has got a glimpse of them. The ending is poignant with a discussion of some recognizably Celtic elements found in Tomas O'Crohan's The Islandman (1937) and other early twentieth century Blasket Island sources. I very much enjoyed it.