I'll be honest and say that my rating is harsher then I would usually do.  I want to rate this book a 4. I really enjoyed it and the world finally had an added level of complexity that I was hoping for in the rest of the series. The reason I want to be more harsh is that I wish that the first three books in the series had a bit more world building to substantiate the ending.  (Don't worry I won't have any spoilers in this).  It almost felt like the author realized that they had some plot gaps and added lore to fill it in.  I don't think that is actually the case. There is enough hints about what was going on in the earlier books to show that there was some intention behind it.  

That being said it is the best spin on Arthurian legend that I have read in a long time and I will definitely be rereading this series again in the future.