Book files are keyed to the serial number of your Kindle. You can give someone a file of a book, but it won’t open for them. I’m sure there’s a similar arrangement for the Mac and Windows versions. There are also Droid and I imagine iPhone versions as well.
I found this out when I had to get a replacement Kindle, and I just copied the book files. They no workee.
You can loan books to other kindle owners which is cool. I haven’t tried the library service where you can check out ebooks for free from your local library.
What you CAN do now is loan a book to someone else. Some publishers allow it, some don’t. Amazon takes it away from you for two weeks, then gives it back.
They’re also trying to work out the details on letting your local library loan you a book. Most of the other formats permit it now, and there are folks all over the country who have NY Public library cards for that purpose. With the difficulty in getting library funding these days, a method by which you could donate your Kindle book and the library could then loan it out to those who subscribe to their service could be a welcome source of revenue. As long as they limit the loan to one patron at a time, it seems like for a publisher it’d parallel what happens with a regular book.