Michael Janich's book,
Street Steel (
http://www.staysafemedia.com/product.php?proid=41&sub_catid=3&page=STREET%20STEEL), is an excellent resource for the person who is contemplating carrying a knife for self defense. I highly recommend this book to anyone who wants to learn about proper selection, carry, and deployment of a knife. I have to admit that I bought knives in the past based mainly on their aesthetic appeal and not to their functional purpose. This book opened my mind to the different possibilities that knives offer. Considering the cost of the book ($24), it is worth one's time to purchase this before one makes their first or next knife purchase. The savings in trouble, aggravation and money no less, make this book an excellent buy.
The main point I got from the book, and it was covered early one, was selecting a knife that worked with all styles of clothing you wear. Michael covered the advantages and disadvantages of fixed blade and folding knives. For me, the folder seems to be the best option for the range of clothes I wear, though a fixed blade neck knife has appeal to me as well.