Having read the book forst, I was disappointed that the conclusions reached within the text were not made on film. The film appeared somewhat biased toward there being a 'race' of Celts, which the book certainly did not, focusing rather on a 'Celtic' culture rather than a genetically-linked people.
The book spent a lot of time investigating and documenting the spread of what we now call 'Celtic' while placing this within a firm historical context. This, I guess, is the benefit of not having to fit findings into a short television presentation which needed to attract an audience.