I recall my dad playing the harmonica often on the weekends when I was young.
He learned to play the instrument all on his own, with no prior musical training.
Since then, several very good introductory books have become available.
Patience and a gradual approach are most helpful in making progress in learning to play this instrument.
The reward is a very personally expressive experience.
There have been some amazing practitioners past and present on the harmonica. Fortunately, there are recordings available of their work.