Answer:
Lovely and round I shine with pale light grown in the darkness a lady's delight.
I have a little pool with two layers of wall around it. One white and soft and the other dark and hard. Amidst a light brown grassy lawn with an outline of a green grass.
Curtail me thrice I am a youth; behead me once a snake; complete I'm often used in truth when certain steps you'd take.
Light me up in backyard gatherings
I wiggle and cannot see; sometimes underground and sometimes on a tree. I really don't want to be on a hook and I become a person when combined with “book”.
I'm a god. I'm a planet. I measure heat.