If you were creating a library and had $500 to spend for each of these subject areas, which reference books (generally, for the lay person, or undergraduate student) would you buy?
Computing (inc. computer science); medicine; astronomy; physics; philosophy; political science and economics; and world history (/not/ USA history)?
The intention is to spend money on classic texts that will last a good ten years or more, and that will be valuable into the future.
So for computing, nothing about Ruby on Rails, Node, or other flavor of the month. But Code Complete might make the cut.
Things which are freely available online, probably also won't make the cut. So in political science and economics, /Wealth of Nations/ and /Capital/ won't be chosen. However, /A Theory of Justice/ might be.
Authoritative works (e.g. in medicine /Gray's Anatomy/) are preferred.