Mundanes who involve themselves in things they know nothing about are likely to meet unpleasant endings.

All Lightwoods look the same to me—

In a closed urban fantasy, the magical world is secret and no one knows about it. In an open urban fantasy, everyone knows about it. So with a closed fantasy, you have to figure out how the world keeps itself secret, and with an open one, you have to figure out how knowledge of magic has altered the world we know.

I think the main thing to remember when writing a novel is to stay true to the characters.

He felt his heart, which no longer beat, contract, and he wondered if there was anything in the world as painful as not being able to protect the people you loved.

Sometimes carrying the burden of an upsetting truth, and hiding it, is actually a gift you give to someone else. You bear that burden, so they don’t have to, in a situation where telling them will change nothing.

My name is Herondale," the boy said cheerfully. "William Herondale, but everyone calls me Will. Is this really your room? Not very nice, is it?" He wandered toward the window, pausing to examine the stacks of books on her bedside table, and then the bed itself. He waved a hand at the ropes. "Do you often sleep tied to the bed?