Packaging is not my “full-time” job but I do get an occasional project. I don’t know what it is, but the packaging projects I get tend to be my favorites. But since packaging is a small part of my day-to-day duties, I don’t spend a lot on reference materials.
I have been eyeing this book for about a decade now, The Packaging Designer’s Book of Patterns , http://www.amazon.com/Packaging-Designers-Book-Patterns/dp/0471731102/ref=sr_1_1?ie=UTF8&s=books&qid=1272548672&sr=1-1
I never wanted to spend the money but I did just receive the book as a gift. It’s a great book, I should have spent the money a long time ago. The 10+ variations on a pillow pack is worth the price of admission. If you know these patterns like the back of your hand, it may not be the book for you. If you are a part-timer like myself, it is well worth it.
Also, I would like to recommend the Graphis Packaging series. They haven’t come out with a new one in a while and the old ones can be dated, but you can get used copies from Amazon for under $10. A cheap price for some great inspiration. Although #4 is very old, I would recommend it because it has many interesting structures. #s 5-9 are mostly graphics-oriented and have little inspiration for structure so I would pick and choose only a couple, you don’t need all five books.