I recently just read Sapphire Blue and Emerald Green back to back in rather rapid succession. SB and EG are books two and three of the Ruby Red trilogy. Unlike some trilogies, it doesn't make sense to only read one. It's not just three related stories that build on each other, it's one really long epic that spans three books.
That said, Gier does a great job of bringing you back up to speed in case it's been awhile between books for you. She does great job of being subtle with exposition and summing stuff up.
I enjoyed this series, even if I didn't exactly understand the titles. Each of the time-travelers is named after a gem and our titular character, Gwen, is "Ruby" but they make a big deal out of her hair not being red, unlike so many others in her family line. And then not sure where the next two titles came from, other than it's a theme of gem+color. But that's all really immaterial in the end.
Why review them together? It seemed silly to separate them, that Sapphire Blue is the one you read after Ruby Red and before Emerald Green. You do not read in any other order and you do not skip a book. That would be bad.
All in all, this is a major epic with a few slow burn mysteries. And it's a fun read. As always, there's the complexity of time travel. They made some specific rules, such as you weren't supposed to travel back to an earlier point in your own life and there was also a maximum you could travel. It worked well because the idea of buildings being around for centuries make sense in London. Were you to set it somewhere in the U.S., they'd quickly be jumping back to vacant lands where buffaloes roam.
The idea is that there are 12 time-travelers in all. They use a device called a chronograph to travel in time, sometimes to perform missions, but sometimes just because they need to. If they don't regularly travel back in time in a controlled fashion, they risk traveling to an unknown time in an uncontrolled fashion. There is, however, risk of encountering yourself in the past (Because you've also traveled from another time) so the secret society keeps meticulous records. It is interesting because it's sometimes difficult keeping all the "this will have happened in the past" and fortunately they don't use phrases like that. One other element I liked was that you determine how long you'll be gone for and that's how long you're absent from the present time. None of this stuff like in Back to the Future where you can jump back to a few seconds after you left - if you're in another time for two hours, you're gone for two hours. Your life continues, just in a different time.
By the third book, there's two big events that a lot builds up to. Oddly, in both cases, it seems like those events end as quickly as they begin, not necessarily worth all the build-up. (Though one is shocking.)
This is a normal girl discovers superpowers and girl gets romantically involved with a boy who may or may not simply be playing her, but because it happens over such a short duration (yes, yes, time travel, hundreds of years, but all within a few weeks), it's quite the whirlwind. So even though our heroine has to come to grips with all kinds of things that turn her world upside-down, she doesn't have time to go over-the-top with her angstiness. In fact I'd say in two instances, she should have more of a response.
But it works. It's a fun trilogy, there are surprises, you have to pay attention and there's also quite a bit of humor. The only thing that surprised me is that her ability to talk to ghosts and demons, while it is an asset to her, it never plays out in a big way where she uses that to her advantage to get her out of a major scrape. It's played more for comedic effect, especially the little gargoyle that keeps asking her to buy a cat. But you'd think that somehow this come into play in a bigger way. But, still, a fun read. Even more impressive when you think it was written in German and translated.
I also like how chapters are separated by quotes about time and life from famous people of history.