SOLVED. A 3 books in one book type. Starts in a castle.

[Brett]

I started reading it about 12 years ago. Red hardcover. Pretty bulky. Only thing I remember reading is that two boys passed notes and they made a secret message in the notes so the cook could not read it. Every letter in between the letter “e” eventually spelled a word. Example: e h e e e l e l e o e = hello

[Brett]

Deltora quests -Emily Rodda