The Library is an abandoned New York Public Library building purchased by a bank Finch owned. Long abandoned, it serves the place where Finch and Reese usually meet and discuss their Persons of Interest cases, as well as their command and communication center when Reese is out in the field. It also contains the books Finch uses to decode the irrelevant numbers the Machine sends him. Its location is a carefully guarded secret, and only a handful of people know if its existence.
Inside the LibraryEdit
Finch keeps most of his technical equipment in two rooms on the second floor of the building, which are secured by a gate and a time lock. A generator serves as the main source of electricity. ("Root Cause")
The main workspace is a big, round desk with several computer screens in the middle of the bigger room. There is also a pin board with all person of interest cases Finch has collected before starting to work with Reese. A huge glass pane now serves for collecting and sorting through information on current cases.
Another small room located right next to the main room is where Finch keeps other equipment that he uses for audio file handling. ("The Fix") As the library has never been cleared of its inventory, there are still thousands of books stored in the shelves. Finch, being an avid reader, often picks books for himself to read. He has also used them to build a playpen for Leila Smith. ("Baby Blue")
Finch used another room in apparently a different part of the building when he first brought Reese to the Library. ("Pilot")
In the seriesEdit
It is located in an abandoned library building that was sold to a bank Finch controls. When the bank declared bankruptcy, the building went into a kind of limbo, disappearing from records. ("Pilot") There are at least two entries to the Library. One inside a tunnel ("Pilot") and another one on the north side of the building. ("Firewall")
Real life locationEdit
The actual property was built in 1908 and formerly belonged to the New York School of Applied Design for Women which became the New York Phoenix School of Design in 1944 and was later formed into the Pratt Phoenix School of Design. More recently the building was used by Touro College until it was sold to a private investor in 2006. 
The building was designated a New York City landmark in 1977. It is located at the corner of East 30th St and Lexington Ave.
Only a few characters have been at the library and know about its existence:
- Harold Finch (owner)
- John Reese
- Leila Smith (although she is only a baby) ("Baby Blue")
- Alicia Corwin ("Firewall")
- Leon Tao ("Critical")
- Samantha Shaw ("Trojan Horse")
- Nathan Ingram (originally used it as a base of operations) ("Zero Day")
- The Library has appeared in every episode except "2πR", "Booked Solid", "Relevance" and "God Mode". In "Super" only the exterior was shown when the Machine was searching for Reese.
- Reese sometimes refers to the Library as "HQ". ("Baby Blue")
- Two of the windows in the main room have a yellow coloured border inside the pane that looks like a yellow square the Machine applies to people who know about its existence.
- Because of a strict policy regarding dogs at Reese's apartment, Bear has moved to the Library. ("Bad Code")