It is described as a scaled model of the city covering almost two miles from Burnham Harbor in every direction. It is about eight feet long and a foot high. It is a meticulously detailed and accurate in its representations 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 miniature 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. After using it, Harry Dresden is usually exhausted.
It is possible for him to be sensed while in spirit form. Also, Dresden is not in touch with his body when he enters Little Chicago, and cannot tell when he is becoming exhausted. He cannot cross thresholds.
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 Dresden's head. Dresden mentions that he uses jeweler's tools to adjust the model.
In the series
In Proven Guilty, Harry Dresden began building it between Dead Beat and Proven Guilty, 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. Dresden was hesitant 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. Dresden spends four hours of ritual preparation before his first attempt at using the model when the phone rings and interrupts him—it's Molly Carpenter wanting to get bailed out of jail.
Before Dresden finally attempted to use it in order to track Molly, Lasciel tried to stop him, terrified it would kill Dresden and therefore herself. Dresden actually gave some thought to digging of the coin for protection but rejected it. The model worked, but the spell was unsuccessful. It turned out that the hair sample Dresden used for the tracking spell was from when Molly was a baby, therefore bearing little connection to Molly as she is. At Murphy's suggestion, they used some of Charity Carpenter's blood to track her daughter using Little Chicago, which worked. At the end, Bob said that it should have blown Dresden's head off but someone had fixed it - not him.
In White Night, Harry Dresden uses Little Chicago to track and Listen to Grey Cloak when he follows him to a meeting with Madrigal Raith. He then follows Grey Cloak down into Undertown, where he encounters the necromancer, Cowl. Cowl, who senses his presence, lashes out with his power. Little Chicago absorbs the energy backlash and results in a melted hole in it — instead of in Dresden's head.
In Small Favor, Little Chicago is repaired. Harry Dresden puts his Order of the Silver Oak pin (which allowed the summer fae Gruffs 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. This frees Dresden to do his job.
In Changes, the model was destroyed with Harry Dresden's apartment by a Molotov cocktail thrown by the Eebs, a Red Court field team determined to kill Dresden and stop him from saving his daughter, Maggie.