There isn't much clarity about data, let alone digital purchases, with estates.
For example, if you kick off, your email provider may be very reluctant to give anyone (including your executor) access to your email. If you leave the password around, though, they don't particularly care who logs in.
So, I'd assume that if you left your account information and password in your papers, someone could (at least unofficially) access those titles. I'm not sure if the ownership can be transferred to another account.
It's possible something is buried in the various agreements -- I'd guess in most cases you cannot officially transfer purchases. But it probably varies from one vendor to the next.
On the plus side, though, you won't be foisting boxes of books onto your loved ones. Do you know how heavy those things are?
One minor tip: Don't put passwords into your last will & testament. Those are public documents, so anyone can read it.