Or at least the pages appear blank... Check out the conspicuously open book for hints. Look at it. Twice.
Tell me more hints
The open book said that there's a technique of invisible ink by using lemon juice. The text can be read by bringing the paper close to a source of heat.
What should I do?
Warm up the pages in the diary. But not too much! There's a lighter on the kitchen table, btw.
How to warm the book?
You can't use the lighter on the book, directly. Use the lighter to light something else up.
Use the lighter on the incense stick to light it up.