I've been meaning to show off my notebooks for a while now. I've been keeping them since I was 18--I started back in 2008 after watching the behind-the-scenes for Pan's Labyrinth and hearing about how Del Toro would carry a notebook everywhere and write notes and draw sketches for his stories constantly. I'd already been keeping sketchbooks, but I hadn't considered keeping notebooks until then!
I carry a larger notebook in my purse (I think they're about A5?), and use it to write... just about anything. Poetry, notes, story notes, notes for decoding cryptic messages, video game notes, panel notes, and so on. It's fun looking back through these things. I only just recently finished the 12th one (which, yes, has a similar cover to my 13th--the 12th one was a misprint and the company sent me the corrected one, but I decided to use the misprint anyway).
A while back I found that Oz passport style notebook and thought it looked neat. I kept it in my desk and used it for random notes for a while until I decided to start using it as a mini-journal--I keep a digital journal that I write in every week, but it's hard for me to remember everything that happened, so keeping a mini-journal to quickly jot down things happening throughout the day helps me write the more detailed entries. I'm only on my second one now.
And... for Christmas last year I got myself a pair of cargo pants so I could have more pockets on me, and decided to start keeping a mini-notebook as well for the times when I didn't have my purse on me! The current one is one of my mom's old notebooks that she barely used (she has dementia, so she... can't really use them anymore, now). I use this guy for EVERYTHING and I LOVE him. I'm about 3/5ths of the way through him right now.
So yeah, I've been using notebooks for a long time (and this isn't even touching on my church notebooks... I don't even know how many of those I have), but only recently did I discover that being into stationery is A Thing and I've been getting into it even more now, learning how other people use and organize their notebooks and learning about the specific sizes and types of paper and pens and so on. It's fun!! I can't believe it took me this long to dive deeper into this hobby.