The World of PostSecret by Frank Warren is pretty much about what the title suggests. There are vignettes about symposiums, secrets, the perfect secret, and even the mail carrier who brought so many of secrets to Frank's house. There are also many examples of postcards and other things that people have sent in to share their secrets.
Some of the secrets are very touching. I found myself getting teared up more than once. Some of the secrets are informative, especially in the way that they inform us we are not alone in the things we keep secret. It was also touching to read the segment on the PostSecret application and the reactions of some people to secrets that they saw posted there.
If you enjoy the PostSecret website or any of the other PostSecret books, then you will like this book as well. It's worth a read. It's nonfiction and biographical in nature.