Like other people, I use GoodReads! You can make collections based on the books you have read, would like to read, or are currently reading. But you can also make other collections like if you have a particular author you like and want to keep track of their books or organize by genre too. Plus you can see reviews about the books you want to read and even leave your own review on them after you finish reading.

I use the Libby/ Overdrive app to get free books through my library but also use it to mark books. The app uses a system called tagging where you can make up tags on your own or use their premade tags already to organize your books. Just like in GoodReads, I mark books I want to read or have successfully finished.