Daisy, I’m so excited for you!
In a secret room, I’d put seasonal things such as warm blankets, gloves, scarves, sweaters, etc. in the winter. A way to stay warm/cook would be another one with plenty of food! A thermal cooker would be really good too. I use mine every week now, but an extra one in there would be great.
The idea of a copy of all your books in there sounds perfect. You make it so affordable with the pdf versions.