“PEAK” is the top of something, usually a mountain.
“PEEK” is a quick, furtive look at something.
So if you SNEAK PEAK, you are stealthily climbing to the top of a mountain. If you are getting an illicit preview of something, that is a SNEAK PEEK.
Please stop getting this wrong. It is beginning to walk up and down on my spine.
Edit: And of course, I get a typo of my own. Pfui! That’s how thrown off I was. Bad grammar is contagious! Stop using it!