There don't seem to be any books about Nvu and the help that comes with it is no help at all.
There is an Nvu Tutorial that you can download: click here
Nvu has a dedicated forum where you can ask questions about using it: click here
Two Nvu 'how to' demos: click here
But I'd suggest that you just play around with it and you will soon get the hang of what is going on. If you want to get to know HTML as well you can click on the HTML tab in Nvu and take a look at what it has written.