Just one. DONT PANIC

When you say "knows" basic mathematics and sciences how basic or exhaustive are you talking? Because without calculators or slide rules you definitely want things like log and trig tables and the methods from which they come from which he probably will NOT know off the top of his head unless you feel like re-discovering hundreds of years worth of stuff. And the way the pyramid of technology works, a book on steam engines is probably useless for building a steam engine. Metallurgy and basic tool making is probably much more important and much more difficult to re-discover.

Have you considered that any up to date book today cannot avoid mentioning computers and thus will be just worth as toilet paper or fire starter in a post apocalypse?

@DKNguyen, include a book of log and trig tables.

@L.Dutch-ReinstateMonica, they can ignore the part about computers in a medical book, for example.

Do any machines, and especially machine tools, survive? If so, their maintenance manuals come just behind the book of log and trig tables.

Few/no machines survive since it is a rural area in a less developed country.

Why would you need steam engines for a "colony" of 250? Why would the people in rural area in less developed country would need to learn from english books? I can assure you that such people have more than basic knowledge on growing plants (an math behind it).

Henley's 20th Century Book of Formulas, Processes and Trade Secrets. The latest edition is from 2010, but I don't know if it is an updated edition. Contains recipes for alloys, paints, fireworks, toothpaste, medicine, cleaning agents and all sorts of other things. goodreads.com/book/show/…

@SZCZERZOKŁY, they are hunter-gatherers, not farmers.

@Galactic There would be important world-building fact about the world itself if you had "hunther-gatherers" who have no knolwedge about farming. And no access to "farmer almanach" in their language. Or the need for H-G to change to farmer after apocalypse.

"Must be up to date as of 2020": I would rather have a book from 19/20th century with some aproximations, rather than a correct one from 21th century, but that is way to specific in his domain and assume you have acces to all modern machines, raw or manufactured materials, ask lot of manpower to do... Basically, the stirling engine may be weaker than a nuclear power plant, but while I could do the first one from scratch, the second one seems innaccessible to me

Location of the survivors is important to know - e.g. I have a book that calendars what to grow, maintain, cutback and harvest each month with a British allotment in mind, which would be incredibly useful if the group were based in the UK but not say in the US as the climate is different. Equally a book on what plants/animals are poisonous/dangerous/edible is generally going to be region specific.

@Kepotx, I didn’t mean the latest technology, I meant scientifically accurate as of 2020.

@Galactic does it have to be real book, or could it be broadly plausible books like "masonry and carpentry technics of the middle ages"

It's important to know what type of apocalypse has occurred. Pandemia of lethal virus scenario assumes there are enough raw materials and common machines. Nuclear war would drop our (almost) all electronics and computers. Asteroid hit would change climate etc.

Why would the majority ("hunter-gatherers") support the lifestyle of this scholar under these circumstances ? Your scholar would have to provide some tangible benefit for such a society to be "paid" in some way by other people's hard labor. Knowing mathematics does not feed you or keep you warm.

What caused the apocalypse (virus, radiation, aliens)? What were the effects of said radiation to the wildlife and environment? Where is said area in which they live? Are there cities nearby or at least a library from which they can get the books? If they're in the equator, learning about the plants and animals would be all they'd need, assuming the forests are still standing (the Brazilian Amazon is so rich in natural resources their original inhabitants never had to depend on agriculture, living predominantly as hunter gatherers to this day)

Wikipedia on am e-reader with a solar charger

If all people have reverted to hunting-gathering, then it is highly unlikely that the survivors come from a rural area. On the contrary, people who ignore agricultural technology in our world tend to be urbanites.