Cross-marketing is a fancy term that's usually applied to product linking. You may not be aware of it, but you see cross-marketing all the time. When the commercial tells you that the cheese spread goes great with this cracker, that's cross-marketing. Coupons from soda that allow you to buy candy is another example. Cross-marketing is everywhere, and you can use a form of it to promote your self-published books.
Finding the Links
You're not a huge corporation and you probably don't have a bunch of industry-related ties, but as an indie author you can still use cross-marketing to push your work. Like every other type of indie promotion, it only requires a little bit of outside-the-box thinking.
Let's play a game. Answer this question: what's your book about? Make a list of words that come to mind when you ask yourself this question. Think about all the different activities and events depicted in your book and place them on the list.
Now use it to cross-promote your work. Link your book to products or activities and use that for marketing. A cross-marketing Tweet, for example, may read "Enjoy skiing? find out what happens when this daredevil hobby turns deadly in Sabotage Slopes."
Is there a lot of food in your book? "We all eat, but in Roman Vacation you'll find out what it's like to eat gourmet meals every day." See how it works? Now start composing your own.
You probably don't have the connections to work out expensive advertising agreements with corporations, but you can still link your work to promotable products and activities. Make your list, find you links...promote your book.