u/Comfortable_Cheek_45

I read this about 6 years ago and I can't for the life of me remember the title. Forgive me due to my deteriorating memory, but i believe this book has/was about:

>A young female main character. A teenager maybe.
>Finds or receives a letter from her dead/missing grandma (?) Might be a relative.
>This leads to her on a mission to find all 10 (I THINK it was 10 letters 😭 it might be 12??) letters
>I don't think the relative was actually dead.
>A genre of mystery
>A cover design that was blue and had an envelope on it?

I remember there being a sequel book. I lived in the Philippines during that time and found it in the National Book Store. Thank you so much!