This was a lot of book for little content. The Moon lights up, but not very well, and the light doesn't translate to any of the interior pages, which were quite dark in coloring (illustrations by Clare Patane, who quite obviously did the bulk of the work involved in creating this story), so I had to wonder what the purpose was, plus the button isn't amenable to little fingers pressing it, so a young child might have trouble lighting that Moon.
That said, the story inside, which was very short and light on text, was quite charming, aiming at showing a child that things which go away (like the Moon appears to), always come back, so it offered reassurance and a touchstone for children who might have separation anxiety.
For me, this story could have been done better, and the lighting didn't seem like it was worth the extra expense and bulk. This was quite a fat and heavy book for a young child to manipulate, and you would not want such a child gnawing on this book because of the electrics and battery. If you can afford to pay for mostly show and not much tell, then go ahead, but for me this seems like it might be an unworthy burden on a family paying all the attendant bills a young child brings with her. In that light (pun intended!), I'm not going to recommend this one, even given how charming and useful the story was.
I think you'd be better off showing your child the real phases of the real Moon. You could take a series of pictures and tell your own story to the same effect and make it a much more engrossing and enveloping story for the child. A better story is that the Moon doesn't actually go away and come back - it's there all the time even when you can't see it! This might offer much more reassurance for a young child than this book does.