.. Reading the diary

Or at least the pages appear blank... Check out the conspicuously open book for hints. Look at it. Twice.

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.

Warm up the pages in the diary. But not too much! There's a lighter on the kitchen table, btw.

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. )

Show the diary to the stick to read it.