The DIARIES OF THE FAMILY DRACUL series is remarkable and appealing historical fiction --- the series falls within the realm of historical fiction because Kalogridis bases it on characters from recorded history. I'm a big fan of hers and have read all of her recent books, and I'm also a huge fan of DRACULA and believe in preserving the integrity of the classics.
THE DIARIES OF THE FAMILY DRACUL series is a completely unique take on the original DRACULA, and readers will find that it really adds a lot to it. Even if you may not have realized there were lingering questions, reading this series will answer them. A few examples of questions addressed are: Why did Dracula commission Jonathan Harker specifically and why did he choose to come to England rather than elsewhere?; who were the three female vampires in Bram Stoker's story?; where did they come from and what happened to them?
This series is a lot darker than the original, if you can imagine that. The story is written in a parallel style to the original diary format, and you'll find that Kalogridis has done a significant amount of research on Vlad the Impaler to create the story. This enhances the classic profoundly. The bottom line is that it's also a thrilling story.
I do have a MAJOR concern with the series in that it significantly alters a main theme of Bram Stoker's classic, particularly the central faith-based concept in Stoker's ideal of good and evil. While I'm not at all opposed to the concept of diversity in religion and acceptance of alternative ideas, I do feel strongly that in the instance of tying into a classic, Stoker's original thought processes should be preserved in every important way --- especially in a story which has become so colossal. But you'll have to read the series to know more because to comment further on this would be a spoiler.
That said, it is a thrilling trilogy, and Dracula becomes a much more menacing and terrifying villain than he has been. I highly recommend this book based on the quality of the writing and the story, but I would not recommend it to anyone who reveres the original DRACULA because it is too inconsistent with the classic.