Amazon recently added the option to have the cover of the book you're currently reading displayed as your lock screen. The new feature is currently available in more recent models of the e-reader and should be available globally over the next month. The feature will support covers for most books, magazines, and comic books you're currently reading.
The update is available specifically for no-ad Kindle devices. It works on the 8th and 10th Gen Kindle, 7th and 10th Gen Kindle Paperwhite, and on 8th and 10th generation Oasis and 7th Gen Kindle Voyage e-readers. You can check to see if your Kindle is compatible here.
You'll be able to turn on or off the new feature in the Kindle's settings menu, where you will find a "Show Cover" toggle for the new setting. This seems like a fun, cool feature but it also makes one wonder why hasn't it always been available?
Lastly, do you really want everyone to know what trashy novel you're currently reading? I don't think anyone cares that I'm currently reading Voltaire's Philosophical Dictionary and would probably think I am being a pretentious show-off. Then again, nobody ever needs to see that I am reading the very saucy The Lover by Marguerite Duras. So there you have it, turn the feature on at your own risk but if you do, pick a title that will garner the least judgment, like a puppy calendar or a cookbook.