Box of Secrets — Raquel Lyon

Piper has learnt three things:

Never trust your parents.
The supernatural world exists.
Watch out for Wednesdays.

Six months after her father's disappearance, Piper takes delivery of a mysterious box, and soon afterwards, a weird-looking boy begins to stalk her.
A hidden letter and a chance meeting lead her to new friends who want to help.
But to get answers, Piper must leave the life she thought to be real behind, and accept that her new friends are not the only ones with secrets; her father kept the biggest one of all.

The Dragonblood trilogy is part of the Fosswell Chronicles. It can be read as a standalone series or as a follow on from the Foxblood trilogy. Box of Secrets is the first book in this set.