If you haven't read The Eighth Day, stop reading and see my review. This may give away details of the first book.
I loved this series. Unknown to "normals," there is an eighth day between Wednesday and Thursday called Grunsday. The people that know about this day are descended from King Arthur, Merlin and others from that time. Some of them are "transitioners" who are able to live in normal days as well as Grunsday. Some are "kin" and are stuck only living on Grunsday. Merlin created the eighth day to trap the kin and Merlin's descendants are the ones keeping the magic of Grunsday alive. There are only two descendants of Merlin left and everyone wants to protect or control them.
That's the basic idea, without giving anything away from the second and third books. Basically, if you like fantasy or adventure, read this series. I loved it!!