Update Nov 1: Amazon Kindle book lending is now up and running in th US.
At the momnt if you buy a book from Amazon on your Kindle reader, it's yours and yours alone. If you want to let some else read your book, you have to hand over your Kindle as well.
That's about to change. Amazon says it will soon allow some books to be lent to other Kindle owners. You'll be able to lend out a book for two weeks.
There are some (important) restrictions: you won't be able to access your book while it's on loan and .... and this is a big AND...it'll be up to the publishers to decide which books can be lent out.
Word has it that the books that will allow lending will be slightly more expensive than books with a restriction on lending.
Comments
http://www.amazon.com/gp/help/customer/display.html?nodeId=200324680
In the Kindle browser settings there should be a button to "change to advanced mode".
Hope this helps.