Would it be 20 Master Plots and How to Build Them? Or perhaps Historic Costume for the Stage by Lucy Barton? One of my favorites to turn to is A Glossary of the Construction, Decoration and Use of Arms and Armor: in All Countries and in All Times, though it is only a starting point for detail.
Would you carry a dictionary? A thesaurus? (or get tricksy and find a copy of Reader’s Digest Oxford Complete Wordfinder: A Unique and Powerful Combination of Dictionary and Thesaurus so you would have both?)
When I look at my bookshelves, and see the myriad references on everything from herbs to shipbuilding, I am really glad that I don’t have to make this decision in real life. And that the Kindle has offerings like What Shall We Do Now?: Five Hundred Games and Pastimes for free if you look hard enough. Great for adding a bit of spice to the plot (or unexpected baby-sitting.)
So, what is YOUR one essential reference you would carry across country?