Introducing SpineAlign. I have seen a few other tools that do something similar, but SpineAlign has a few features that I think help set it a bit apart. 
1) Hold SpineAlign from the bottom to push seat books level with the edge of a shelf.
2) Use the short end of SpineAlign to start a 35mm wide section at the beginning of a shelf or finish a 35mm long section at the end of a shelf with a consistent 10mm offset. 
3. Use the long side of SpineAlign to drag-align all the books on a shelf with a consistent 10mm offset. 

Great for books, DVDs, video games, anything with a consistent edge on a shelf.

I included the *.step file and if you use OnShape SpineAlign can be found there as well. Have fun!