I don't keep a list.
I recently started a "freebie" list because I bought a book and then later found out I had already downloaded it as a freebie but never read it.
I can recall books I've bought, but it seems not ones I've gotten for free, so I now I have a list I can check before I buy.
(Hopefully I *like* the book; I don't mind buying a book I will like, but it will be disappointing to have paid to read a book that I didn't enjoy and could have read for free.
)