Start with this book

Exposing harmonica to young children or adult just comes naturally — is good for brain development. Early harmonica training helps develop brain areas involved in language and reasoning. Recent studies have clearly indicated that musical training physically develops the part of the left side of the brain known to be involved with processing language, and can actually wire the brain's circuits in specific ways – says Saket Jalan, certified music therapist from the World F...

I bought this one or two months ago. This is a very good book for almost anyone,
from a complete beginner to skilled. It tells you many things about this little instrument.
Everything is explained using the 10 hole C diatonic harmonica. After learning this
you can apply these ideas to other diatonic harps too.

great book like most in the dummies series.