I have serious reservations about the academic value of this book. I thought this work looked promising at first sight as a concise overview of paganism in several areas and forms. However, although the book is split into areas such as Greek, Roman, Celtic, Germanic, the Baltic and Russia, it is sometimes unclear as to which period of time the authors are referring in these areas. I also get the feeling that "paganism" is treated an homogenous whole over the entirety of time and space, which is not true.
More worryingly, and I found this is quite typical in the work, in the 'Baltic Lands' section, the authors talk briefly about the role of sacred groves in Finno-Ugrian paganism without citing any primary sources, any secondary sources or mentioning what period of time their paragraph relates to, until suddenly they inform their reader that in the nineteenth century sixty-four groves remained.
The lack of citation of primary and secondary sources is an outstanding flaw of the work. They do have endnotes but their use of them is wildly inconsistent and frequently unhelpful. Unfortunately this lack of citation also transfers itself to the bibliography, where they do not bother to list their primary sources on the grounds they are available in many editions. This in itself actually makes it more vital to know which translation they are using. And translations it must have been, unless the authors are able to read a wide variety of dead and modern languages, which I suspect is not the case.
This lack of care in documenting source material is no where more evident in their section on 'The Later Celts'. Here they recount the story of Martin of Tours inspecting Northern Gaul and destroying pagan shrines and trees. They tell the tale of St. Martin being challenged by the pagans to perform a miracle: he could cut down their sacred tree if he stood under it as it fell. According to the authors, St. Martin declined their challenge and went elsewhere. Their citation for the story is not from a primary text, but rather from a secondary book, which they got in another book, which cites something else - and that does not appear to be a primary text either. Alas Jones and Pennick, this is not the version of the story the rest of the world interested in Martin of Tours knows! In the "Life of St. Martin of Tours" written by Sulpicius Severus, who begun his "Life" while St. Martin was still alive (he died in 397AD), the story is related clearly and in it Martin does stands under the tree, but rather than in it hitting him the tree miraculously sweeps round and almost squashes the pagans, which is more like it for a Saint's Life. There is no excuse for this kind of error or poor citation. I have just checked, and even the information on Wikipedia could have saved them!
So, read it this book if you must, but keep in mind the need to double check their citations for everything they bother to give references for, and to check and find sources for the many things they do not.