The gentlemanly @onechaptermore, generously mistaking my enthusiasm for expertise, asked me for some thoughts on book collecting. This seemed a little big for Twitter, but a great idea for a blog post.
So, my advice, such as it is, for book collectors:
First, figure out why you're doing this.
And, speaking as a die-hard bibliophile, "because I can't imagine not" is a perfectly acceptable answer. But maybe you see them as long-term investments, like wine or stamps or something. Or you're after a quick profit - eBay, dealing, etc. Or you love an author, his or her books express your inner philosophy and you need them all for you. All of these reasons are great, but they will impact what sort of books you're looking for and what condition they're in - new, used, signed, inscribed, etc.
Second, pick a theme.
I chose "theme" not "topic" deliberately, because it can be something more intangible - a category that may only be specific or identifiable to you. It'll matter whether you pick a tight theme, say, the works of Joe Abercrombie or a broad one, e.g. "grimdark". The benefits? Well, with Abercrombie, you can achieve it. Despite his best efforts, there's still a finite amount of Abercrombiana. The idea of completing a collection is kind of cool, if slightly harrowing the instant a new book comes out.
With a broad collection, you'll never finish. That can be frustrating, or fantastic. The broader the theme, the more likely you are to find something for your collection: every flea market, Forbidden Planet trip or lazy eBay browse will reveal more stuff for your ever-growing shelves. (I'd also warn against going too broad. Collecting, say, "fantasy" is dangerously woolly. You'll not only never achieve it, but you'll go broke trying. Boundaries keep you sane.)
From personal experience: I stumbled on two of Maxim Jukobowski's Black Box Thrillers - just as reading copies. Then I found a third. Then I did some research, and learned there were only nine. So, the quest began, and, within about a year, ended. Awesome. Satisfying. Now what? Fortunately, I'm also after Fawcett Gold Medals, and, at last count, there were an infinite number of them. Whew.