Could anyone explain to me the meaning of the abbreviation Nagel's?

It has to be the travel guide (as Yosef Baskin pointed out in comments); I don't have a copy, but it looks like the equivalent of our contemporary Lonely Planet.

Dad and Uncle Christopher used to do something similar with their beloved Rand McNally Atlases. They'd argue about some place or another up in the Catskills (NY, USA) and only end when one or the other would say: "Let me get my Rand McNally..." From there it was all downhill, as they say, and they might end up an hour later discussing the hill topography/geology anywhere from Skyline Drive in VA to some obscure corner of the Ozarks!

Note that in the case of Nagel's, the possessive ('s) is part of the title proper.