Very well written, but...
...why drag Atlantis in? If a sinking continent is needed, then the Celtic myth and legend Lawhead is drawing on has plenty to offer. Llyonesse, for example, but Lawhead picks that up and plonks it somewhere around Devon/Dorset, as a new home of Atlantean refugees.
I suppose the Atlantean refugees give a "real world" basis for the Fair Folk, but why, when Lawhead includes druids and their magic?
Disproportionate rant aside, this book is a rollicking good read, with the all-pervading sense of impending doom fictional story in a fictionalised ancient time a very real, very modern, very relatable feel.