Once in a great while, I dare to look through my credit card bill and account for my spending habits. I generally spend around $150 a month in book purchases of one kind or another. This doesn't usually bother me in the least since I use the library as much as I possibly can - particularly since they started up their ebook/audiobook app! But just in case I do feel a twinge of guilt, I have my go-to thoughts that help me out:
1) Lots of people rent or go to the theater to see movies. In this mode of entertainment, they get to spend anywhere from $3 (rental) to $20 (theater) in order to enjoy a 2.5 hour piece of entertainment. I generally buy ebooks, bargain audiobooks, or used paperbacks costing between $2 and $8. These provide me with 10+ hours of entertainment with the possibility of repeat reading later. Very economical fun.
2) Books make me healthy! Read this for the evidence.