Notebooks! All the notebooks!
This is assuming you don’t mean books. Assuming that hoarding books is as natural as breathing for anyone who reads (and who actually has space) and that using books would be a cop out for that reason.
So my answer is notebooks. I have far, far too many. There are some people who only buy a notebook when they need one. Others who might grab a couple and keep them on hand. I have probably close to 200. And that’s just a guess because I’m not counting.
To be fair, not all of them are large. Some are super tiny, like index card size, and some are even smaller, talking you could string them on your keychain if you wanted, or stuff them in your pocket. I have one shelf that is two layered deep in unwritten-in notebooks because it’s shorter/smaller notebooks in the back and an extra layer of those super tiny ones in the front.
I’ve got spiral-bound ones, glued ones, some with locks, some with leather ties, some with recycled paper, some with glitter. I’ve even got one with a furry cover. Some were super cheap. A few not so much. Many were presents. It’s a go-to gift for me for a lot of people because they know that notebooks are always appreciated no matter what size/shape/style, especially so if they’re pretty.
Whenever I get knee-deep in a new novel idea or start a new challenge or just need to hit the refresh button on my mind, I’ll go sift through my notebooks to find one that fits the idea I have just right.
The one for my Houndmaster books is a floppy green that used to have a tie but it broke because I used it so much.
The one I’ve set aside for my shaman stories if I ever get to them is a pale blue folded cover with wood rods keeping it shut.
The one for Canvas Blues is a spiral-bound plain tan one that had hard covers so I could write short ‘yesteryear’ pieces whenever I was out and about.
The one for my poems was a gift. Feels like leather, but is probably fake, has a cute lock-clasp and a stone embedded on the front cover.
The one I used (but haven’t touched in years) for my DaSunder Chronicles is a smaller gold mottled, hard-covered notebook that reminded me of the desert.
It’s an obsession. But I guess it’s better than say, food wrappers or nail clipping or anything equally gross.