Using "N" value for cohesive soils is a very complicated matter. For the non-cohesive soils (I mean non-plastic) I may introduce a suitable reference as follows:
Karl Terzaghi, Ralph B. Peck, Gholamreza Mesri, 1996, "Soil Mechanics in Engineering Practice", 3rd edition,
Chapter 9, Page 402, article 50.2.8.