Concerning individual books vs. a group, you could do both.  I have a lot of products that have content thumbnails for each individual element in the scene, plus another one that is a "preset" scene with all the elements arranged into a scene that's ready to use, and that works well for me.

  Personally I like the ability to break a scene down into it's individual components (ie, a single book) because that allows me to arrange the scene however I want; maybe I don't want a line of books on a shelf and I want a single book, or I want a pile of books on the floor, or I want a staggered stack of books, etc.  But it certainly would also be helpful to have an entire row of them if I did want to stock a library or something.

 

  If you decide to use the .duf DAZ Studio format, it should be very easy to do.  Just import your object, get the materials working the way you want, then File > "Save As" > "Scene Subset".

  (If you save as "scene subset", that creates a file that will be added to an existing scene when you load it.  If you save as "scene", that creates a file that will REPLACE the existing scene, which you probably don't want.)

  If you wanted to create a single book and a row of books, you could just put the single book into one file like Book.duf, then make the row of books and save that into a second file like BookRow.duf.

  As a DS user, while I do sometimes use products distributed in .obj format, it generally scares me off because in the past too often something was wrong or materials are missing when I import it, so I've mostly stopped trying.  I'm probably missing lots of good things because of this.  Unless I'm the only one like that, you may find additional customers simply by offering it in a different format.