No part of saying "this is how much I'll spend on books each month" makes any sense to me. You don't budget; you just buy. The more books the better! Fill up every bookshelf in the house and when you're done, buy another bookshelf.
A couple years ago, I made the biggest mistake of my life: I got rid of all (most) of my print books in favor of Kindle. What can I say? I was taken in by that minimalism doc on Netflix. Fast forward to today, and I lament not having a bookshelf...