"'Librarians don't create new worlds,' he explained. 'They discover existing worlds.'
Thanks to NetGalley for the ARC.

When I saw the description of this book, I got super excited because it reminded me a bit of The Story Thieves by James Riley, but an adult version.
The concept of the book was pretty original and intriguing. It takes place in a world where people known as Librarians can enter and exit books at will (similar to what takes place in Story Thieves. There were aspects of it that also reminded me of Cassandra Clare's Shadowhunter series. While I did really like the concepts, the worldbuilding was vague in some places. The only two "stories" the characters visited were fantasy books---which is fine on its own---but they were very similar to each other, and considering the wide variety of genres available, I thought it could've featured more unique stories.
As for the plot, I found the pacing to be almost too fast in some places. There were some times when scenes would shift within a sentence and if I wasn't paying extremely close attention, I could get lost. Overall, the plot itself was intriguing, as it involved Emerson coming to terms with her past, learning about her powers as a Librarian, and choosing between the "official" Librarians in one branch and the "rebels" in the other. I also didn't realize it was a series, so I was surprised when it ended on a cliffhanger.
And the characters? Personally, I felt that they weren't fleshed out well enough to be enticing. There were a lot of side characters that I couldn't remember the names of and that were introduced once and forgotten about. I found Silas to be creepy and honestly I didn't like Adrien that much either.

Spoiler

He kept bringing up his kiss with Emerson, which was just weird to me. I did like him better than Silas though...

. Emerson was an okay protagonist (I tend to have issues with female main characters who are "different" but I didn't mind her), but she wasn't a Jude Duarte or a Katniss Everdeen.
My only other comment would be that the book felt more YA than adult, and I'm not sure if it was intended to be or not, as the characters all are older in age.
This book was an enjoyable read with a unique premise, but I didn't find it to be a favorite of mine.