Norton Motorraeder 1902-1998 by Jan Leek is nice from the picture point of view, but not a source of impartial, historically correct information.
To be fair to Leek, I know authors for this row of books- there are others on other marques- get very little money for their work, so tend to put as little effort as possible into them.

The pictorial material shows mainly reproductions of Norton sales material, with a heavy bias towards the late featherbeds, Commandos, and Rotaries. Leek drew on two sources, the pre-rotary material coming from one German archive that is obviously good on the 1950 through 1970s, the other source for rotaries and the C652 "International" models being yours truly.

In his text, Leek pays his dues to all German popular prejudices against British motorcycles, and shows scant understanding for the demise of the industry, and for what went on after the 1960s. 
All German motorcycle journalists, following the lead of the late "Klacks" Leverkus, the "pope" of all aspiring writers in the 80s,  wrongly place the downfall of the British motorcycle manufacturers in the early to mid-1960s, ignoring the fact BSA got the Queens Award to Industry in 1970!

The book is a nice adition to a Norton archive for the pictures, but do not buy it for the text- and should your German be limited, see that fact as a bonus.