Learning to Play Like a Whistler
This is a great tutorial for someone with no experience playing a musical instrument or reading sheetmusic. It starts out by teaching the absolute basics (how to hold the whistle, blowing your first notes, the basics of tonguing, etc.). It teaches the notes (and their values) a few at a time, then you practice several tunes that only the notes that you’ve learned so far. As you progress in the book, you’ll add more notes and learn more about music notation.
This is a very good system for learning to play the whistle (and for learning to read sheetmusic) without feeling at all overwhelmed.