If you want to learn more about a particular industry -- and ideally discover some companies worth investing in -- consider looking at a source many investors ignore: industry trade magazines.

One industry that interests me, for example, is fast food. Over the years, I've learned a lot from my subscription to QSR -- a trade journal named for the (warning: euphemism ahead) "quick serve restaurant" industry that it serves. Just recently, in its April issue, I learned that:

  • Some fast-food locations, such as a KFC franchise in Miami Beach, are attracting business from Muslim diners by offering chicken certified as "halal" (i.e., prepared according to Islamic rules). (KFC is a division of Yum! Brands (NYSE:YUM), along with Pizza Hut, Taco Bell, and others.) This could be a growth driver, if others follow suit.
  • Taco Bell is developing breakfast offerings, and other chains are looking to beef up morning sales. Burger King (NYSE:BKC), for example, has a new Breakfast Value Menu, featuring items that start at $1.00.
  • Wonder what new flavors you might find at your local eateries? A Tyson Foods (NYSE:TSN) chef suggests that Indian fare might soon spread, due to its one-pot meals and vegetarian dishes. A ConAgra (NYSE:CAG) bigwig sees a growing popularity for Spanish flavors, along with pomegranate, caramel, and roasted garlic. A Sara Lee (NYSE:SLE) chef points to Southeast Asian flavors such as satay sauces, lemongrass, kaffir limes, and Thai basil.

Periodicals like QSR can also familiarize you with still-private companies in the industry that you might want to keep an eye on. Cold Stone Creamery, for example, has earned some high marks in various QSR ratings, and In-N-Out Burger is also a well-respected and growing chain. Knowing about a company like this can help if and when you hear that a bigger company is interested in buying it. Similarly, if you read about how Mexican food is America's second-favorite ethnic cuisine (after Italian and before Asian), you'll perhaps nod knowingly when a big food company buys or launches a Mexican food chain.

You'll find a directory of many trade journals at Yahoo!, and some sites also offer free subscriptions. There are all kinds of interesting magazines about airlines, advertising, hotels, publishing, computers, health care, shipbuilding, textiles, travel, energy, apparel, cars, banking, and more.

Of course, there's a bit of a downside to trade magazines -- they keep you focused on one niche, and they're not always objective. To counter this problem, consider trying out one or more of our own newsletters, which deliver our best ideas to you monthly. Take a free trial and let them introduce you to some compelling companies in industries you hadn't considered before. And then you can dig deeper via trade journals.

Longtime Fool contributor Selena Maranjian owns shares of Yum! Brands. Try any one of our investing services free for 30 days. The Motley Fool is Fools writing for Fools.