A Darker Shade Of Magic by V.E Schwab
As has been said by numerous other reviewers I am sure, the world building in this book is exquisite! I don't think I've seen the multiple world's idea done quite as well as I have here. It's certainly the most thought out and crafted, and would be hard to beat.
Both the main characters were will defined and their relationship is one I could definitely believe.
The one draw back is the plot reliance on the mcguffin. By the end it definitely had a feel of the Ring from Lord of the Rings, and ultimately I felt it was this that held the book back from being really great.
I look forward to continuing in this world with this charactera in the future.