In death note, he puts a piece of paper in the door frame and if it falls he knows someone went in his room. What’s a similar tactic I can use on my diary to know if someone even opens it?
Apart from keeping the diary in a locked drawer, here are some steps you can take.
- Put the diary in a drawer, close the drawer and then pull it back open just a few millimetres. You can tell if someone else has opened the drawer and then closed it all the way.
- Position the diary so it is 1 cm from the edges of the drawer. Then you know if someone took it out and replaced it.
Leave a bookmark half way across the page. This could be a separate paper or card bookmark. Naturally the person reading the dairy will open it at that place, and when they close the diary, the bookmark will be in a different position.
Finally, you can implement something like the door trap you mentioned. Place a tiny object on top of the diary, that does not stand out visually, for example a circle of paper that was stamped out by a hole punch (for a ring binder). If anyone picks it up, or opens it, that will fall off. If they notice it fall, they won't know the exact place on the diary where you left it.
Use a tiny bit of glue and stick the corner of two pages together. Maybe before those two pages write something like: "account information", and then an unglued page filled with fake secrets, and then the glued-together pages. Use super weak glue, and they will break it, to look at your secrets. Maybe glue the middle of the outer edge of the paper so they have to break it to see. Essentially, the seal-on-letters concept. Use sticky substance to close a page, and if it is open when you come back, it has been tampered with.
Or, you can eliminate the person completely (https://www.youtube.com/watch?v=VNIF4pXsgt0)
Use colour-changing powder and coat with resin. This will only work short-term, and only if they do no submerge the book/their hands in ice water to cover/prevent their tracks.
Put dust on the book, if it is suddenly shiny one day, either someone touched it and wiped it, or your roommate/spouse/parent dusted the book.
Security cameras, maybe not affordable, but an option
If the person you are guarding against is a librarian, dog ear some pages, and then if they are un-dog-eared when you come back, they have seen it and been infuriated.
Light sensor in book (don't know how to implement)
Hang the book using string, tied at the opening end. Measure the "excess" of the tie, or cut it to be exactly some length If it has been untied and retied, you will know by the excess being different. Or, use a very obscure and hard to tie knot.
Put graphite powder between some pages. If it is smeared/spilled, someone touched it