Books are more available now than they have ever been at any time in history. Thanks to the internet, we now have access to the inventories of more new and used bookstores than any one person could browse in a lifetime and to the collections of individuals that they are willing to share or trade. Beyond that we have more books, both new and old, available in electronic form. Plus we can easily search the collections of libraries around the world, many of which will allow us through our own local libraries to borrow material using inter-library loan.
You can get pretty much any book you want.
Given that, what guidelines do you have for why you should keep a book in your house?
Here are mine:
- I'm going to read or re-read it within four years.
- I consult it at least once a year.
- I periodically re-read or consult it and it has annotations in it which remain useful to me.
- It is an object of beauty and/or sentiment which brings me frequent joy.
- I wrote it. (Don't have any of these yet besides the bound copy of my senior thesis, but I'm workin' on it!)
Decide your guidelines and then look to see what you have that doesn't meet any of them. Donate those replaceable (or never needed again) books to the library or charity. Give the stuff you love more room and give yourself less unnessary weight of stuff in your life.
Discardia 
#4 has become my motto for keeping things. If it’s not frequently useful (like say a blender), I ask myself if it brings me joy to have it. If the answer is no, I get rid of it.
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