Little Chicago

Little Chicago is a model of the city Chicago, made and owned by Harry Dresden.

Design
It is described as a scaled model of the heart of the city ("almost two miles from Burnham Harbor in every direction"), about eight feet long and a foot high. It is a meticulously detailed and accurate in its representation of everything, and is thaumaturgically connected to the city via pieces of real buildings, trees, etc. that have been placed on it. He also included miniture versions of the ley lines going through the city. It is used by him for various magical purposes.

In some ways the model can be used like a focus, assisting tracking spells. But it also allows a sort of remote viewing where the user is spiritually placed in the actual city. He is able to move around, track people, and listen to conversations, but is unable to physically affect the city in any considerable way. The model can also protect the user from a psychic backlash: when the necromancer Cowl discovered that he was being spied upon, the backlash he created burned a hole through the model instead of Harry's head.

The downsides are that it's possible for him to be sensed while in this spirit form. Also, he is not in touch with his body when he does it, and cannot tell when he is getting exhausted.

History
Harry began building it after Dead Beat, when he started to have extra money from his new warden's salary. According to Bob, he worked on it every day for six months before it was ready for a test run, which happened in Proven Guilty. He was hesitent to try it out, because it contained a massive amount of energy, and could possibly blow up if there was a flaw anywhere in the design. He finally attempts to use it in order to track Molly, near the middle of the book, but is unsuccessful.

In White Night, he uses Little Chicago to track and listen to Vittorio Malvora. He follows him to a meeting with Madrigal Raith. He then follows him down into Undertown, where he encounters the necromancer Cowl. Cowl, who senses his presence, lashes out with his power. Little Chicago gets a large hole in it.

By Small Favor, Little Chicago is fixed again. Harry puts his oak pin (which allowed the summer fae to track him) inside of a bag of catnip, suspends the bag over the model, and allows Mister to play with it. Whenever the bag touches a part of the model, it sends out a corresponding tracking signal, forcing the gruffs to run all over Chicago.

During the events of Changes, the model was destroyed with Harry Dresden's apartment.

Who fixed it?
''I was working in my lab the next day, trying to make notes of all that had happened so that I wouldn’t forget anything. Bob sat on the table next to me, helping me with the details. “Oh,” he said. “I found something wrong with Little Chicago’s design.” I swallowed. “Oh. Wow. Bad?” “Extremely. We missed a transition coupling in the power flow. The stored energy was all going to the same spot.” I frowned. “That’s… like a surge of electricity going through a circuit breaker, right? Or a fuse box.” “Exactly like that,” Bob said. “Except that you were the fuse. That much energy in one spot will blow your head off your shoulders.” “But it didn’t,” I said. “But it didn’t,” Bob agreed. “How is that possible?” “It isn’t,” he said. “Someone fixed it.” “What? Are you sure?” “It didn’t fix itself,” Bob said. “When I looked at it a few nights ago, the flawed section was in plain sight, even if I didn’t recognize it at the time. When I looked again tonight, it was different. Someone changed it.” “In my lab? Under my house? Which is behind my wards? That’s impossible.” “No it isn’t,” Bob said. “Just really, really, really, really, really, really difficult. And unlikely. He would have had to know that you had a lab down here. And he would have had to know how to get around your wards.” “Plus intimate knowledge of the design to tinker with it like that,” I said. “To say nothing of the fact that he would have to know it existed at all, and no one does.” “Really, really unlikely,” Bob agreed. “Dammit.” “Hey, I thought you loved a good mystery, Harry.”'' (General discussion and popular theories about this are to be found here.)

Why is there a tarp?
"A long table in the middle of the room was currently covered by a canvas tarp."

There is no given explanation for why there is suddenly a tarp on the model in Turn Coat, which is strange because there isn't one in Proven Guilty, White Night, Small Favor, or even later on in Changes. It is pointed out that Little Chicago could have been used to find Thomas, in the same book, yet was not mentioned even in passing (though one could also argue that it was implied).

As Harry describes his blasting rod in Small Favor as being "like a shape covered by some heavy tarp" when he tries to think of it, and also mentions having painful migraines in chaper one of Turn Coat, it is posited that someone mentally compelled Harry to forget about its existance, briefly, for some unknown purpose.