Sapphire Blue, by Kerstin Gier ( Henry Holt and Co, 2012, YA), is the sequel to Ruby Red (my review), and you absolutely have to read the first one first, or you won't have much clue what's going on. However, Ruby Red is lots of fun, so there's no reason not to read it!
The basics of the plot: There's a secret cabal of time travellers whose genes whisk them back in time--to prevent surprise temporal whisking, they have to "elapse" in controlled time travel quite often. Gwen thought that her snotty cousin Charlotte (one of the most objectionable fictional Charlottes going) was destined to be the one who got the gene in her family...an assumption shared by all the other members of the society. So when it turns out to be Gwen instead, it comes as a shock....
There are lots of secrets to this mysterious organization that Gwen isn't being told, and a backstory of betrayals and intrigues that she's becoming ever more involved with (rather relevant backstory, because the thing with time travel is that the characters can go back themselves and become part of it). Gwen is supposedly destined to bring about some sort of milestone viz the secret cabal, and there are, apparently, rouge time travellers who left the cabal who don't want this to happen.
And then there's the complication of Gwen falling hard for Gideon, a young time traveller who might, or might not, have been involved with vile cousin Charlotte--incredibly handsome, full of secrets, and kind of a jerk, what with all his blowing hot and cold.
Keeping Gwen sane as she come to terms with all of this, and travels to the 18th century to meet with the sinister head of the cabal, is her good friend Lesley--sharp as a tack, an ace at Internet searches, and less preoccupied than Gwen (whose mind is full of her Gideon dilemma, she takes on the Mystery Solving aspects of the story. There's also James, the 18th century ghost lurking around Gwen's school, whose good for a few quick lessons in 18th-century life skills. And in this book, Gwen is adopted by the demonic spirit of an ex-gargoyle that only she can see-- a useful, though tricksy, companion....
So all this takes place in just a few days, with the result that the books are full of detail and conversation and many happenings, and a tad short on overall plot advancement. Fortunately, I found it all very enjoyable, so the lack of progress toward actually getting any clear answers to much of anything didn't bother me in the least!
The time travel, in terms of experiencing the past, is not actually a central element in the book--yes, Gwen travels back to the past, most memorably getting so tipsy at an 18th-century soiree that she entertains the guests with a song from Cats, but the time travel mostly serves to introduce Gwen (and the reader) to various characters and plot elements. So this is one to read for the entertainment value of its central character struggling to solve the mystery of a. her destiny, and the whole secret society thing and b. her relationship with Gideon, and not one to read for educational visits to the past....which, of course, is just fine.
(Just as an aside--Sapphire Blue was originally published in German, and translated by Anthea Bell, the go-to girl for German to English YA fantasy translation. I have read a number of her translations, and I think she must be very good, because each different book she's worked on has a different feel....)