Eligible Kindle books can be loaned once for a period of 14 days. The borrower does not need to own a Kindle — Kindle books can also be read using our free Kindle reading applications for PC, Mac, iPad, iPhone, BlackBerry, and Android devices. Not all books are lendable — it is up to the publisher or rights holder to determine which titles are eligible for lending. The lender will not be able to read the book during the loan period.
Not bad for a first effort. Hopefully over time the restrictions (which, despite being mostly in Amazon's economic interests, likely originated almost exclusively with the publishers) will be relaxed a bit. I did the math on my Kindle collection, and just 8% of my books currently can be loaned out. I wonder though if more books are "eligible" (i.e., the publishers are OK with their being lent), but just haven't yet been pushed through the proper channels.
With the current model, it isn't hard to imagine that the three-week limit ("You have seven days from when you first received your e-mail about the book load to accept the loan. Once you accept, you have 14 days before the loan expires") will require a fair amount of coordination between parties so that the loan isn't wasted. For example:
Lender: "You have to check out this book I'm reading!" Borrower: "Yeah?" Lender: "Yeah, I should be done with it later this month." Borrower: "Hey, are you done yet?" Lender: "I just finished it. I'll loan it to you in a few minutes." Borrower: "No, not yet, I won't be able to finish it within three weeks if you send it now. Send it on February 18th."
Absent that conversation, you likely are putting a lot of pressure on the borrower to find time to start and finish a book within 21 days.
(Relatedly, consider this idea from Kiran Max Weber.)