Jackaby short story-- read anytime after you've read the first book
Jackaby surprises Rook with a birthday treasure hunt and, naturally, hilarity ensues.
This is listed as book 1.5, but as long as you read the first book first, so you know the characters, you're good.
Read it whenever.
I read it right after reading Ghostly Echoes, because that was when I realized it existed.
If you haven't read the first Jackaby yet... what is wrong with you?
Go. Read it. Now.