@gabby: Regarding collections, you might want to check out this tutorial on Marvin's blog:
http://blog.marvinapp.com/post/62249442864
The section you want is "Smart collections".
If your friend is new to Marvin, she might find Marvin's index of all tutorials useful. Here's the link:
http://blog.marvinapp.com/post/74284911817
Right now, there are no plans to make Marvin auto download books from Dropbox. Many people seem to keep 10s of thousands of books in their Dropbox and if they turn on auto download it'd cause all sorts of problems. I'm still frying to figure out a smart way to deal with this.