Calvin and I discovered a series recently called The Missing, by Margaret Peterson Haddix. There are currently 5 in a planned series of 7 books. I am not going to tell you anything about them because I personally loved going in totally blind and wondering what in the world was going on! They are adventure and history and science fiction all rolled into one. The first book was especially well done, but Calvin has read all five and found them all very exciting, and I have enjoyed the two I've read so far.
They are upper elementary/middle school books with a brother and sister as main characters, so really for boys or girls.