This is a great book for anyone in a long-distance relationship. It's a relatively short, easily read handbook with very useful, practical tips. Specifically, the book contains over 350 creative and romantic activities long-distance couples can use to help strengthen their relationships while they're apart. The best part is that you can usually find it on amazon for less than $7 or you can order the booklet version straight from the authors for as low as $2.50 (depending on how many copies you want)!
Here's a sample page from inside of the book: