Most information about the Nakasendo is, unfortunately, in Japanese. There is limited material in English about the old highway, but more about other highways in general and, in particular, the Tokaido highway.
Regarding the Nakasendo, the earliest Western writing is J.J. Rein, Der Nakasendo, in German. Following on, Satow and Hawes have a comprehensive guide which mentions every post-town on the Nakasendo and their characteristics (Ernest Mason Satow and Lt. A.G.S. Hawes, A Handbook for Travelers in Central and Northern Japan (Kelly and Co., Yokohama’, 1881). Three Rolling Stones in Japan also follows the Nakasendo, but the literary merit is dubious.
In Japanese, the best source is the Rekishi no michi chosa (Survey of Historical Roads) which was initiated in 1978. Each prefecture undertook its own survey. On the Nakasendo, Kyoto, Shiga, and Tokyo prefectures declined to participate. The other prefectures through which the Nakasendo passes, Gifu, Nagano, Gunma and Saitama, have produced very comprehensive accounts of the original route and historical artifacts along the wayside, all in Japanese.