Amazon purchase memory

I love Amazon.  I buy everything there from books to groceries to electronics to clothes.  Being something of a homebody it's very convenient.  I particularly like when they remember what I bought and when.  For example, I can answer the question "how long did that last case of tea bags last me?"

However, they remember books you've bought only by type.  So if I bought the paperback version of a novel a year ago and then see it for the kindle, Amazon appears to have no clue that I already read it after buying it from them.  I find that very annoying.

I know.  I should be able to remember what I've read and what I haven't, but I read a lot of books and have burned out quite a few gray cells over the years.   I started this list just so I could go back and see if I'd already read something, and even if I liked it.  It helps but I think Amazon could do better.  After all, they're using computers and I'm stuck with my aging brain.