Lucy and the Merman

One day, Lucy discovers a merman. You'd think that would be the start of an exciting adventure, but this particular merman is a married, older man with male-pattern baldness who repays Lucy's help with an introduction to his family, a little tour of his house, and a nice night at a music festival. Except for creepy vibes from all the merfolk, nothing ever really happens. The end.

I remember flipping through this book as a kid and looking at the pictures, but I don't think I ever read it. That of course didn't stop me from holding onto it for fifty-plus years.