There are at least 3 different parts of amazon, with different rates and terms and such.
If you order direct from Amazon and chose the free super save shipping, they will ship the books when they have a low volume and it may take over a week for them to ship, and then you have the normal shipping time for USPS Media Mail (a week or more).
http://www.amazon.com/gp/help/custom...&nodeId=527692
If you ordered from an Amazon Seller (such as Target, Borders, etc), they set their own shipping policies.
http://www.amazon.com/gp/help/custom...nodeId=3848941
Since you mentioned used books, you probably ordered from Amazon Marketplace which has a mixture of small businesses and individuals. The sellers promise to ship within 2 days of when you place the order. If you used Domestic Standard shipping (4-14 days) it most likely uses USPS media mail.
http://www.amazon.com/gp/help/custom...?nodeId=537734
You are right that normal first class mail doesn't take that long, but Media Mail is much cheaper and slower.