can i publish a book using downloaded content of people and scenes?
It really depends on the content. Almost all of the content sold in the DAZ 3D store can be used for commercial projects, such as publiching books or even animations. The renders (images) that you produce are yours, but the figures used to make them are not.
Some free items from ShareCG for example, have a readme with them which forbids commercial use. Alwasy check the EULA and readme files for any items that you use.