Book collecting is one of the greatest hobbies there is. It combines beautiful, interesting objects with the excitement of the hunt and, who knows, maybe even the possibility of making some money! Worst case scenario—you wind up with a lot of books. There’s no way to lose.
First, figure out why you’re doing this...Second, pick a theme.
I chose “theme” not “topic” deliberately, because what you collect can be something more intangible - perhaps even a category that may only be specific or identifiable to you.
Wow, I'm all over the place and that isn't even everything. It would do me no harm to get a bit more deliberate in my collecting. I have bought a couple of books from the Top 100 SFF list, and no doubt that theme will grow on my shelves, too.