For me, it's a printed copy of von Neumans "First Draft of a Report on the EDVAC", the groundwork for modern computer architecture. Bought it from Kronecker Wallis, a tiny Spanish publisher.
I barely know anyone who would understand why I treasure it, let alone own a copy.
The book is literally a series of messages you can transmit by radio to an alien intelligence somewhere else in the universe so you can establish a language in common with them. It starts off by teaching numbers: ., .., ..., i.e., sending 1 pulse, 2 pulses, 3 pulses, and so on, teaches numbers to an extraterrestrial being. From this it creates arithmetic (".. pause ... longer-pause .....", means 2+3=5), and then creates abbreviations for symbols, and goes on the build up a lot of mathematics, and then the concept of time, and basic physics, and even very abstract ideas like "good" and "bad", though I've forgotten how it managed to encode that (but I recall it seemed plausible at the time I read the book).
The book is impossible to find now. I'd love to find a copy of it.
A wonderful insightful look into 1882 thoughts about what constituted history. The human interest stories are the best - how the names of local townships were selected, the storms and railroad schemes, fights and murders.
https://www.goodreads.com/book/show/13019115-the-name-s-phel...
No idea how I came across it but it is an account of pre-independence Ireland that is lucid and fun.
If anyone's interested,
My dad owns a rare Buddy Hall biography called Rags to Rifleman. Buddy Hall is one of the greatest pool players ever to live.
Almost nobody has it but that's kinda part of how it's supposed to be.
My favorite one is an East German guide to useful trade-oriented words in eight different languages. Basically it's a bunch of columns with words like "consumer goods" or "deck cargo" in German, English, Czech, Russian, etc. The introduction especially has some interesting lines like:
"This book is mainly intended for business with capitalist countries."
The Tibetan Book of the Dead
The Ascent of Rum Doodle
My Dad has a lot of old late 1800s early 1900 books including a huge 2 volume dictionary, which is actually more of an encyclopaedia. It has topic sections like "How to defend yourself from savages" or "How to dig your own coal mine" etc.