I watched the tv series first, loved it and then read the books. I still think the adaptation holds up well and there are inevitable losses from any source in order to fit a serial format but some of the changes are baffling. The congregation story line is brought to the foreground for no apparent reason; in the books a certain T.J Weston episode did not happen, Peter Knox was not killed by Sarah, while neither Satu nor Domenico are half as prominent. Further, New Orleans is only a background element in the books while New Haven is cut completely from the adaptation. Worse, characters like Corra and Philippe who are fundamental in the books - are rather diminished. Corra doesn't appear at all in season three despite being crucial to the location of the BoL and to Diana understanding her own identity. Philippe, who essentially bookends the narrative (plus engineering events through time and beyond the grave) in some ways....is also wholly absent in season three. I get that SFX budgets may be a factor here (for instance there's much more magic in book three than is visible on screen in season three) but certain changes do feel very arbitrary. Some of the casting is really spot on tho', particularly Ysabeau, Philippe, Sarah, Fernando, Father Hubbard, Miriam and Gallowglass. Stephen Cree IS Gallowglass and I sincerely hope he eventually gets his own series! The leads are lacking a bit of edge IMO, there's much more push and pull between their very different natures in the books - it's more heightened/problematic and less smooshy - but this is also down to writing.
Overall a solid effort and most enjoyable (esp seasons one and two) but season three and book three are very different beasts and I think too much of the magic (ha!) is lost.