Library selectors are a lot like retail buyers in that they have make important decisions about potential trends long before the trend emerges to profit from the increased customer interest. "50 Shades of Grey" is a good example. Saavy library systems were buying from the Australian independent publisher long before it was picked up stateside so that they had it on the shelves when the customers were asking rather than weeks later.
The question often becomes, "How do I know what will be popular?" That is the magic of a great selector! I've seen people who do it well and others who just get by. You need to be "of the world" to get good intelligence to inform your selection practices. You need to know what is popular on television, what is new in the theatre, what people are buying and what social trends are hot. You need to learn this by going outside, meet people, ask questions.
A colleague pointed out this site to me - Edelweiss: Reading Above the Treeline. It collects all current publishers' catalogues for easy access. You could certainly look at multiple catalogues to determine current trends as publishers are also looking to capitalize on the current interests of readers.
Good luck and spend some time doing something other than reading!