I am not a fan of pruning ... I always try to first talk clients into putting the right-sized plant in the right place. Some plants, however, always outgrow their projected size, so selective pruning is a necessary art.
For most, pruning is indeed a mystery, though it need not be if you turn to the latest edition of Cass Turnbull's Guide to Pruning, 3rd Edition: What, When, Where, and How to Prune for a More Beautiful Garden (Sasquatch Books, 2012). Turnbull is the founder of Plant Amnesty, an organization devoted to timeless pruning techniques and good plant management.
Her classic book has been updated with info on 20 additional plants and three new chapters. Although Turnbull is from Seattle, Washington, most of the plants in the book are ones that you'll find all over the country. It's one of the easiest-to-understand pruning books out there, with easy-to-follow illustrations and clear, concise instructions on what you need to do.
Add this book to your reference shelf ... you won't regret it. And it would be a great present for all homeowners who are determined to try pruning themselves.