Anyone who loves books and reading will likely face a time when the number of books owned is greater than the shelf space available.
Some turn to culling, some to creative stacking, some to building walls of books.

Derby Bookstore, Salem, MA
Faced with my own outsized collection over a decade ago, I vowed to start limiting my personal library to ‘favorites.’ Ah well–while I have indeed met books I haven’t liked, I do meet a lot of ‘favorites–right now.’ Sometimes I appreciate their inclusion on my shelves as a nod to personal history; sometimes they end up sacrificed (donated, of course) for greater quality.
If space is my issue, one might think e-readers are the solution. Oh, if only it was so easy! I enjoy paper reading; the heft of a book, the smell of the pages, the unconscious sense of pacing I have from pages turning and the physical space before the end. After a year of use, I don’t dislike my e-reader as much, but I still prefer paper for favorites. Lendable, keepable, available when the zombie apocalypse hits (because I’m going to need reading material, naturally), paper remains my favorite.
So what’s a reader to do? First, there is a somewhat small ‘must-buy’ list (Peter S. Beagle, David Quammen). There aren’t many authors on that list; even my favorites have embarked on stories or series that don’t appeal. But there are some series (Kate Daniels, Inspector Chan; The Girl Who…; Vlad Taltos, Claire DeWitt, Peter Grant) that I simply must have (my Preciousss).
My other main strategy of limiting myself was to switch entirely to the library for first reads. I recognize I have access to an unusual resource: not only does my city library system have nine branches, but the library has aligned with a seven-county library system and 52 member libraries. Even better–they have computerized databases with free interlibrary lending. I feel very fortunate to live in such an area. Do I want to read a book? Their catalog is my first stop. This also has the benefit of being one of the least resource consuming methods of reading, so I feel like I get green points as well.
Once in awhile I’ll fail to find a book in the library system, and it will go on a ‘buy-if-I-want-to-read list.’ Interestingly, one of the first books I put on that list eventually became available when I happened to randomly re-check a year or two (cough, cough. Obsessive, who?) later. I borrowed, it became one of my favorite reads of the year and is now on the to-buy list (Cursed). Sometimes a book makes it off the ‘buy-if-I-want-to-read,’ sometimes not. Last time I bought off that list, one went to the library with the note, “you don’t have this in the library system; feel free to keep or sell” (Monster Hunter Alpha) and one I kept (Geist). I tend to donate my mass market new paperback editions, not ones that are worn. I had tried to resell at the local used bookstore, but the downside of my book-loving city is that sell price for even a new book is rather ridiculous (I believe I was offered fifty cents for MHA). Other books that I’m tempted by and have reasonably good experience with the author, I might buy if I happen to run into them in at a used bookstore (Habitation of the Blessed by Valente).
Once I read a book and it passes my nebulous ‘to-buy’ criteria, I make an attempt to decide paper or hardcover. Hardcovers are books I want around as long as I am–I’ve discovered after a few decades that paperbacks just don’t wear as well. For instance, I have a paperback (bought first) and a hardcover To Say Nothing of the Dog. Paperback–lendable. Hardcover–no, just my Preciousss. Depending on my interest in the author, I’ve allowed myself to go to more extraordinary lengths in the past couple of years to get a hold of hardcovers, recognizing I’m committed to the book. Peter Grant and Detective Chen are two series I’ve ordered from overseas, gladly paying exchange and shipping costs. For some reason, the second in the Peter Grant series was extraordinarily hard to find, so I kept an eyeball on Amazon’s network of dealers, with condition and costs. That time, I ordered a ‘good’ copy for something like forty-five dollars, but ended up disputing what the dealer considered ‘good.’ Mostly I keep a haphazard eye on bookseller discount lists or occasionally peruse the hardcover section of the used bookstore. Vlad Taltos will often end there after being put out in hardcover. Occasionally I’ll add more borderline books to my library this way (The Midnight Mayor).
Occasionally at a used bookstore, I might pick up a book I think of as more ‘classic,’ but not necessarily one I’ll endlessly re-read (Snow Crash, Altered Carbon). These also act as my emergency reading back up (because–of course–a committed reader needs a supply laid in against scarcity), particularly for e-reader failure during airplane rides. Then there’s the e-reader. I still can’t get used to the pacing of it. I’ve mostly saved it for advanced reader copy (The Girls at the Kingfisher Club), books bought specifically for traveling (Black Arts), or books that are ridiculously cheap that I’m fairly sure I want (Emperor Mollusk, The Merchant and the Alchemist’s Gate), books that are ridiculously cheap that I might want (The Chronicles of St. Mary’s). So occasionally the e-reader acts like my mini-library, with screening access to a number of books I will likely be interested in. To me, the idea of an unread book in my space–physical or virtual–is a kind of indirect reading pressure, a task to be completed instead of a present to be opened. It is one of the main reason that I refuse most reviewer-copy or ARC offers; I want my reading to be a pleasure and not another item in a task list.
What’s your book use strategy?