Aside from literature, which I love, I have found that I have quite a fondness for owning textbooks, both physical and digital. (Reading them is a whole different story, though)
I have a textbook for Non-Classical Logic, Rock-Forming-Minerals, both of which I don’t even begin to comprehend as I lack the fundamentals, General Physics, General Maths, Zoology, Lichen, Insect Morphology (didn’t even know about that one anymore), Invertebrates excluding insects, Freshwater Biology, a bunch of species identification keys, Inductive Statistics, etc. etc.
Some of those I obviously need for university, but most are stuff I just found for free somewhere and took or downloaded with the intention of learning something.
I always have this scenario in mind of society collapsing, but it won’t be a problem for me because I have a lot of things to learn. Does anyone here have a similar “condition”? Haha
That depends also on the topic. For mathematics, for example, textbooks never really get out of date.
For zoology, botany, etc. it doesn’t really as well, as they have been established fields for a long time. There might be taxonomic changes and scientific name changes over time, though.
The most “susceptible” to obsolescence of my textbooks is probably the “Molecular Biology pf the Cell” one, as that is a field where there’s a lot of changes and new insights still.