...can be found here:
http://www.nero.com/eng/support-nero9-manuals.html
If you are using a previous version, say, 8, change the 9 to 8 by first using the above link and then, on the address line, change the 9 to an 8 and click Go.
Addendum: The page is also useful in that, by clicking on the link for each of the Manuals, one may read a brief summary of what each of the roughly 20 programs within Nero 9 offers.