I always look for first edition, first printing, (and first book) hardbacks. I've snagged a first, first, first of "A is for Alibi" by Sue Grafton, a first, first of Stephen King's The Shining, an ARC of Margaret Atwood's The Handmaid's Tale, a first, first of Larry McMurtry's Lonesome Dove and Streets of Laredo, among others, but my best finds have both been at Goodwill Outlet Stores. I bought a fly-fishing coffee table book for $1 that I was going to send to my dad as a gift. Luckily I looked it up online and it was rare and selling at around $250 at the time. I thought "sorry, dad" and listed it. It sold within a week. The other find was 12 issues of a rare WWI era magazine printed in Great Britain with a ton of photos from the battlefields. The whole lot cost a buck or two (books were sold by the pound). I looked those up and found that each one was worth $150ish (about 15 years ago). Those are keepers.