There are instances when I pick up where I think that I left off with a particular author and I discover that I have already read the book. Or I begin reading a new book and discover that I missed a book in between the one I am currently reading and the one that I last read. This seems to happen with series, and if an author has more than one series it can get really confusing!
My solution is to keep a book journal and there are many ways to do so. Some people like to keep it organized by author or series, writing down everything that they have read by a certain author. Other people use their journal as a checklist writing down all books that they are interested in and then checking them off as they read them.
I try to write down everything I read. Most times I am able to write a short summary of the book and I what I thought of it but at the very least, I try to write down the title and author. I keep my journal in chronological order, if I forget the title of a book or what the book was about it helps jog my memory to see the title of the book that I read before and after that one.
Discover which type of book journal works for you! Happy Journaling!