Now that Kindle Unlimited is out, I updated the post on borrowing Kindle books to qualify for Whispersync pricing. It's linked in the OP.
How to Borrow Kindle Books to Qualify for Reduced Whispersync Pricing
One of the best features of the Kindle ecosystem is
Whispersync for Voice. This feature allows you to sync qualifying Kindle ebooks with corresponding Audible books. The big plus here is that the audiobooks can be purchased at a reduced price (sometimes, greatly reduced).
To qualify for this reduced pricing, you must first own the Kindle ebook. Besides buying it, you can also borrow it to qualify. Amazon considers three forms of borrowing as ownership (Kindle Unlimited, public libraries, and the lend/borrow feature). The fourth form of borrowing (Kindle Owners' Lending Library) does not qualify.
How to Borrow Kindle Books from