This sucks since the books are not cheap. You can lend a paperback, you should also be able to lend e books with their guidelines. I personally don't feel as though I got my money's worth because I didn't enjoy the book and I recommended my friend not buying it but she wants to read it. It would be nice to be able to share it.
I just called Amazon about the same thing. They said the publisher can, at their discretion, decide what can be lent and what can't be. Amazon said they have no control over what the publisher does. However, the 'lend this title' button should be disabled when it happens. Apparently, in the case of the Fifty series, it is a glitch that the button even appears. I asked where it is stated when you buy a book that a title in lendable or not. Amazon said there is no notice of lendability but he would forward my comments on to the appropriate people. I am irritated because I paid full price for these books and should be able to do with them what I want.
I was told by Apple that you could not "lend". In order to use Amazon on my Apple and my IPad, I had to download the App "Kindle" to my IPad then go to Amazon and buy the Kindle edition and download it to my IPad. I got as far as pulling up the edition on my IPad but it doesn't allow me to add it to my Cart I can Add to Wish List...... Now I have to find out how to go further ????? I should be able to open up my wish list and pay for it then download to my IPad. What do you think? Can I do this?