This is the second of the Repeating History series by Dakota Chase. I loved the first one (Eye of Ra), published in 2010, and I kept checking back to see if there were sequels. At last, my prayers have been answered! (Okay, that was a bit over the top, but still... yay!)
The basic premise of the series is that two teenagers at a private school break into their history teacher's office as a prank and accidentally cause a fire that destroys several historical artifacts. It turns out that the teacher is really Merlin, living incognito, and he decides to punish the boys by sending them back through time to retrieve the artifacts they destroyed (while hiding their crime from the school, so they won't be expelled).
That bit of hand-waving aside, the author does quite a bit of research on the time periods involved, and represents them very realistically without drowning us in minutiae. This book is somewhat darker than Eye of Ra, but the characters are also better drawn. I criticized the first novel for shying away from obscenities (they're teenagers!) and Grant and Aston being a little too hesitant about their developing attraction. But though this book is certainly NOT laced with obscenities and sex (Seriously, no, that isn't what I'm saying!), they do occasionally swear, and they're handling their relationship about the same as many teenage boys would - lots of kissing, in between bouts of insults and teasing. The developing friendship/romance is highly entertaining.
I've started the third book (Mammoth!), but thanks to life intervening, I haven't finished it yet. I'll be sure to review it, when I do. So far, I highly recommend this series.