We haven't emailed books before, but I agree it might be desirable.
You have maybe a problem with the PG terms of use in that some Amazon email addresses cause the book to be sent by 3G, which costs the end customer real money, i.e. distribution costs, whereas you can also choose to email to a slightly different email address which only sends via wifi, i.e. for free.
After all it is not PG being user-unfriendly but Amazon.
See feedbooks.com for an example of a website which is reasonably "user friendly."
The solution is quite easy: Ask Amazon to implement a function in Silk that lets user save books to the "Books" tab. Make lots of people ask.
I asked back in 2011. Amazon promised to do it (not to "Books" but rather "Documents" so that it would be compatible with the other Kindles.) But they haven't done so. Good luck.