Picture book with almost watercolor like illustrations of pink and grey. Much whitespace. About a child (boy/girl don’t remember) and he/she looks into the reflection of the water (lake?) and fairy land is on the other side. He/she falls in and is trapped and cannot get home? Read in mid/late 80s.
Some version of The Water Babies, maybe?
There’s a similar setup in The Silver Nutmeg by Palmer Brown. You might want to see if other details match. (It looks like his books are being reissued now. While I don’t see this one reissued yet, just be aware of this if you are looking for specific illustrations.)
Mirrorstone?