A marvel from yesteryear:
Among my father's effects from his maritime days I found this little gem, a pocket-sized compendium of math tables, weights and measures, conversion tables, currencies, and even including biblical weights and measures and a list of "Kings and Queens of England since the conquest."
Nowadays, of course, "there's an app for that," albeit without the feel of a worn and softened hand-me-down booklet. It's amazing how detailed and informative this is in such a small package. The odd thing is, in a book that's as dense with data as this is, there's no publication date. Even the title page says "copyright" without giving the date either of the copyright or the publication.
If you sought to come up with such a pocket-sized compendium of the most useful info for a traveler in today's world who finds himself without a smartphone or access to the 'net, what would it include?