Relatively cheap thrills
With regard to personal book collections, librarians fall into two camps: the "Need to own because I love them" faction and the "I work at a library, for heaven's sake!" cadre--that's my group. A veteran of numerous relocations, I mentally calculate weight and space requirements for every volume I encounter.
Still, I make exceptions to the borrow-not-buy policy without regret. Owning copies that you can lend is wonderfully empowering. You can deliver something fabulous to another reader without having gone to all the trouble of writing or publishing. I recently loaned out my copies of Nick Bantock's original Griffin and Sabine trilogy to a co-worker and am enjoying his appreciation of those elaborate letters, stamps, and postcards far more than I deserve to.
Penny Vincenzi's Spoils of Time trilogy further demonstrated to me that acquiring a whole set of something is a fundamental human need (also that investing in books pays off in the long run). I sent the first book, No Angel, home with my mother after her Christmas visit, figuring that she would revel in the gossipy family saga with British historical setting as much as I did. It's a lengthy tome, and I planned to offer the other two if that prediction proved correct. I neglected to follow up, though, and later Mom mentioned that she'd gone to great lengths to find and purchase them. I inherited the Thrift Gene from the previous generation so view that move as the ultimate compliment for Ms. Vincenzi.
In the future, then, I will always package sets together. At present, I am contemplating the thrill of buying Richard Russo's new book. The "I don't want to rush so someone else can check it out" impulse and the Thrift Gene are deadlocked over this one.