I've purchased most every sourcebook and module.  However, there's not an easy way to find out which ones I haven't purchased.  I even clicked on one that I already owned and it would allow me to buy it again - for myself (not just the gift option).

I don't want to double purchase but would like to ensure I own all the content.  Please let me know how to filter.

I put the books I have into one pile and leave the books I don't have out. o.O

the app is the easiest way imo...just scroll down - 'go to marketplace' are the ones you don't have.