You can send books to your kindle over USB, and I do that all the time for larger books that are above the size limit on the email system.
The big problem is that Amazon no longer allows you to download books from their site to your desktop, so you have no way to actually get a purchased book and send it to the kindle even over USB. However, if you buy non-DRM books from other book sellers you won't have this problem.
> The big problem is that Amazon no longer allows you to download books from their site to your desktop
I've bought a number of books on Kindle that were explicitly marked as being sold without DRM. Does this mean I've lost access to any DRM-free downloads that I haven't already backed up?
They block you from doing this if you're not logged in (as I discovered after wiping and rooting one to give to a friend recently).
As evidence, note that instructions for rooting them requires the device to be registered - this is because it won't be accessible over USB until you do so: https://kindlemodding.org/jailbreaking/WinterBreak/
So if you can't log in...