Having watched the show then read the books, I found that the series cleverly kept all that was interesting and removed the stuff that should have been edited from the books. If I had gone to the books first, I would not have read Shadow of the Night, nor The Book of Life and not watched the series, consequently missing out on one of my now favourite shows.
The conversations between Gillian and Diana in the tv series were not true friendship, more a subtle nervous testing of the waters. You could tell that Gillian and Diana wanted more from it and were trying because the former wanted the kudos of getting a witch from a powerful lineage to join the Oxford witches, while the latter wanted a friend. Both failed because Gillian wanted Diana to be enthusastic about witchcraft and Diana wanted a friend who didn't judge by their prejudices. You can see this with Sean later on, she tries for a friendship, but retreats the minute he expresses disgust about people still believing in witchcraft.
The show also portrayed the story from third perspective, a view which would have made the books better, as the first person narrative of the books tended to go off into tangents - yoga at the Old Lodge, totally unneccessary - and at times failed to convince me, especially when Diana says she's loves him. In the show I was convinced by their relationship, but it took until the end of the second book for me to believe their union would work. When the books had that rare third person narrative, those chapters were more eloquently written.
This series is one of the rare exceptions where I don't mind deviations from the books, because it is so well acted, subtly and eloquently screenplayed, with gorgeous cinematography. It is only because the show was so good that I turned to the books and to the history involved, but then I do love history.