The problem is that there isn't one "Paganism" and therefore there isn't a book that discusses its history. You'd have to find books on each of literally thousands of ancient Pagan faiths. The same basically holds true for Traditional Witchcraft as well as most Witches kept their practices very private.
If you'd like to read about Paganism in the British Isles. I would suggest the following books:
" Pagan Religions of the Ancient British Isles " by Ronald Hutton
" Pagan Britai n " by Ronald Hutton
" Stations of the Sun " by Ronald Hutton
"Pagan Europe " by Nigel Pennick
" The Pagan Celts " by Anne Ross
" Dying for the Gods " by Miranda Green
For a history of the Druids try:
" Blood and Mistletoe " by Ronald Hutton
" The Druids" by Peter Berresford Ellis
And for a history of Wicca try reading:
" Triumph of the Moon " by Ronald Hutton
" Witchfather - A Life of Gerald Gardner " by Philip Heselton
" Wiccan Roots " by Philip Heselton
"Gerald Gardner and the Cauldron of Inspiration " by Philip Heselton
A book by Scott Cunningham was mentioned. While Scott is a great place to learn how to practice Wicca it is not at all a good place to learn about the history of Wicca and it says nothing about Traditional Witchcraft or historical Paganism.