And, to be sure, this is about Logos books or resources or whatever they are called. 
I have been a "hoarder" of books. I remember well, my shelf library and all the books I had in it. And now in Logos my books are growing and take up some "digital" space, small as it is. And the cost of tera bytes is cheaper which allows me to be more of a "hoarder" of digital books and such.
Recently I purchased the NTSK. I have the "old" TSK. Should I delete it out of my library? Keep it for back-up? Or just don't worry about it? What would be your recommendation? Thanks.
Edit: The reason I ask is that these "extra" digital books do cost something, especially when Searching, re-indexing and such....