In comments, Roger asked an interesting question: given all these books I keep acquiring (and, therefore, have to shelve somewhere), how do I figure out when to put items from my collection out to pasture? (Or, at least, out to the free books table.)
1. As you might expect, we begin with denial, as I hear my books sobbing at the very thought that I might no longer want them. How could I be so heartless? So cruel?
"Because there are books stacked on top of books here," I tell them, firmly.
OK, I tell myself firmly.
2. Moving on past the Agony of the Books. Some books I never intend to keep. As it happens, these are the books I now buy in electronic format--SF or mystery anthologies, for example. I generally discard mysteries unless they're Victorian (I have a use for those), although I did hang on to all of my Dalziel and Pascoe novels. I also quickly discarded all those Anne Boleyn romances, and the Dracula knockoffs I currently have stacked up every which way will also decamp whenever that article finally puts in an appearance.
3. As an academic, I get free copies of teaching editions. There are times when one contemplates seven different copies of Jane Eyre and decides that there are other things that could be on one's shelves.
Now we're into the tougher decisions.
4. Question #1: Will I ever read this? For some reason, I built up a rather large stack of postapocalyptic novels. And yet, despite my naturally pessimistic nature, I then found myself deeply unmotivated to read any of them. Out they went.
5. Question #2: Will I ever read this again? At the risk of forever destroying what exists of my geek/nerd cred, I found that the Lord of the Rings trilogy and the Silmarillion fell before this question like orcs in sunlight. (Let's just say that I think you need to have "caught" Tolkien, like Lovecraft, at a certain age; as it happens, my own immune system proved too strong for Tolkien's prose when I finally sat down to read it.)
6. Question #3: Will I ever write about this? Given my line of work, historical novels tend to survive this question, but a lot of contemporary fiction (especially contemporary fiction that didn't grab me the first time around) disappears into the ether.
7. Question #4: Will I ever teach this? Since I do teach the second half of the British novel survey, contemporary British fiction has a good chance of coming out alive. Literature in translation, which comes in handy for some lower division courses that fulfill GE requirements, also has an edge. And I generally hang on to novels that rewrite other novels and/or Shakespeare, as they're helpful for intro to literary analysis.
8. Question #5: Will I ever cite this? Culling monographs is a bit dicey--I just wound up rebuying (for less than a dollar, thank goodness) a book I discarded about a decade ago because I'd never used it, only to discover now that I need it for more than one project. Because interests are not predictable. (...Dracula knockoffs? Really?) These decisions were easier when I was a graduate student, and bought a lot of books that looked interesting without considering whether or not they were actually useful for my scholarship. A random interesting book should be checked out of the library; a useful book should be on one's own shelf.