Reading slumps feel like a loss of appetite and are usually a logistics problem wearing a costume. Three causes cover nearly all of them.
Cause one: the pile is judging you
An unread stack in your eyeline turns reading into an obligation. Put it somewhere else. Keep one book out.
Cause two: you are still finishing the last one
A book you are 40% through and not enjoying blocks everything behind it. Abandon it out loud, to someone, so it stops being a pending task.
Cause three: you picked something worthy
Slumps end faster with a thriller than with a classic. Read the disreputable thing. Nobody is marking this either.
The fastest fix
A short book, in a single sitting, on a day off. Finishing something is what restores the appetite — not choosing better.