Garmin no longer supports, nor provides information or a download link for nRoute; however it is still available from third party sites.
As it was always offered by Garmin as a free product, I guess this is legit.
nRoute works on maps already installed in Mapsource.
Nroute info and nRoute download 105Mb. Note: nRoute 276 was the last version released by Garmin.
nRoute Manual download (1.3Mb). I think the manual auto installs in C:\Garmin\Nroute\nRouteManual.pdf
Some newer maps (e.g Garmin Topo Aus), do not work with nRoute, however there are work arounds. Garmin Topo on nRoute solution tks to R_G_B here