The 'Mail Me Art' project called out to artists to apply their artistic talents to the outside of an envelope, which was then to be sent through the postal system - rather than art for art's sake, mail art for mail art's sake.
The book makes an interesting survey of contemporary artists/designers/illustrators whose work errs toward the slightly alternative. There are brief interviews with 17 of the contributors, which are interesting, but could have been more so. I felt the questions that were asked (the same of everyone, rather than personalising with regard to artist's work, etc) restricted the potential answers somewhat.
I didn't find this book quite as interesting as I had hoped - overall, a bit disappointing, but definitely still worth a look. You should also check out 'Envelopes: A Puzzling Journey through the Royal Mail' by Harriet Russell.