It isn't rare, and it isn't valuable, but in one of my boxes somewhere I've got a copy of
this Volume 2 Prima Guide for Diamond and Pearl. The front cover is ripped and cut, the edges are in
terrible condition, my god, and I think one page is just completely ripped out of it. It's just that it was like,
the prized possession for me when I was younger. It was something I'd constantly reference as I became more familiar with hundreds of Pokemon, their movesets, their types, any evolution methods, and what they were weak to or what I should expect from them. I brought it to school a few times, a teacher of mine took it away and threw it in the trash, and I went in there to retrieve it the same day.
As I grew older and that sorta thing became permanent knowledge I started relying on it for maps of each area to memorize every floor of every cave or find anything in a route I'd miss.
I could argue it's not even that sentimental, since I've stopped using it over time and it was given to me by someone I'd rather never speak to again. I suppose I do have good memories with it, though, and if I have to be honest, I doubt I'd have continued to play or care about Pokemon if I didn't have it to rely on.
I have a second copy of the same book that I keep in good condition. The pages feel smooth to flip through, everything is readable, and it looks fantastic. But like hell it'd ever be the first one I'd grab off the shelf.~