Amazon has exercised its power to control what happens to Kindle books by revoking Lendle's API access.
Update: API access now reinstated.

Original news 22nd March
Update 23rd March - see end.

Lendle is one of the Kindle book lending services established when Amazon introduced its Kindle book lending facility at the end of 2010. It has just had its API withdrawn suddenly and without any explanation.

Up until now Amazon had appeared to, if not support, at least tolerate these services which enable owners of Kindle books to lend out their titles just once and for only 14 days and to borrow titles offered by other people on the same terms. Amazon's only previous move to control such sites was when it forced the Kindle Lending Club to remove "Kindle" from its name.

By withdrawing its API Amazon has effectively taken away Lendle's revenue - from commission on sales of Kindle books from its site - but in so doing has alienated a community of Kindle owners who are fervent book buyers as Lendle only allows borrowing from club members who also have titles to lend. 

Jeff Croft, a Lendle co-founder, explained that Amazon's action came as a nasty shock with no prior notice and no obvious route for negotiation:

“The letter we got from Amazon comes from a “no-reply” e-mail address and offers no formal way to dispute the revocation, or ask for more clarity. We have sent a response to any relevant Amazon e-mail address we could, but as yet, have not received any response from them."

Croft is determined to continue with Lendle:

“We intend to do everything in our power to continue to serve our amazing community. Part of that, of course, is doing whatever we can do get our API access back. Failing that, it’s still very possible for us to run a lending site without relying on Amazon’s APIs. It may take us a bit of time to rebuild, but one way or another, we’ll continue lending eBooks. This only strengthens our drive to bring the publishing industry into the 21st century, even if we have to drag them kicking and screaming.”

The fact that this has happened to only one of the Kindle lending sites suggests that it isn't its operational model that Amazon objects to but rather that Lendle has infringed Amazon's possibly draconian Terms of Service.

One suggestion is that Lendle crossed the line by accessing users' entire Kindle library (lendable or not). But until Amazon explains why it took this action only Amazon knows what the crime was and whether it is prepared to reinstate the API if Lendle agrees to conform to its terms  and conditions in future. 

IIt is obviously in Amazon's own interest to support any service that promotes sales and by their very nature Kindle book lending clubs are just such a service and Lendle's only crime appears to have been over-enthusiasm to its cause.

Update: 23rd March

Amazon has reinstated Lendle after extended press coverage and users tweeting that the only reason they bought a particular book was because of Lendle. The only condition was that they remove the Book sync feature which is in breach of the API access terms.

More Information:

Kindle Lending Club forced to change name
Kindle lending club - how can that work?

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