The United Nations is three things at once: a bureaucracy you can describe, a set of legal instruments you can read, and a running argument about whether it has ever done what it was built for. Most confusion about its record comes from mixing them up — from blaming the Secretariat for a Security Council decision, or the Charter for a member state's refusal. So this order does the description first.
An Insider's Guide to the UN is the orientation: who does what, how a resolution actually passes, what the Secretariat is for, how the agencies relate to the centre. It is not a history and it is deliberately first. Jussi Hanhimaki's Very Short Introduction is a shorter alternative if 272 pages is more than you want.
What it is, and how it was made
Act Of Creation: The Founding of the United Nations is the San Francisco conference of 1945 in narrative detail — the American drafting, the Soviet demands, the small states' resistance to the veto, the wiretapping of allied delegations. Stephen Schlesinger's book shows the Charter as a negotiated bargain between great powers, which is the fact that makes the veto legible as the price of American and Soviet participation rather than as an oversight.
Then United Nations, Stanley Meisler's readable chronological history from 1945 through the mid-1990s, secretary-general by secretary-general. It is dated at the end and it is still the most digestible narrative spine; read it to have the sequence of crises in order before the argumentative books rearrange them.
The institution as an argument
The parliament of man is the sympathetic standard assessment, by a major historian of great powers. Paul Kennedy goes organ by organ, is honest about the failures, and concludes that the UN is worth more than its critics allow because the alternative to imperfect coordination is none at all.
No enchanted palace is the revisionist case, and a genuine shock after Kennedy. Mark Mazower shows that the founders included Jan Smuts, the architect of South African segregation, and argues that the organisation was conceived partly to preserve empire in a new form — the anti-colonial UN of the 1960s being a later capture by newly independent states. It is 232 pages and it will change how you read the first stage. Historians do not agree on how far Mazower's case goes; the two books are best held together rather than ranked.
Human rights: the Declaration and its afterlife
A World Made New is Mary Ann Glendon on the drafting of the Universal Declaration, centred on Eleanor Roosevelt but properly attentive to Charles Malik, P. C. Chang and Rene Cassin — a document negotiated between genuinely different philosophical traditions rather than exported from one. Human rights and the uses of history is the sceptical counterpoint: Samuel Moyn argues in 155 pages of sharp essays that human rights did not become a serious political force until the 1970s, and that treating 1948 as the founding moment is a story told backwards. Read the two in that order and the disagreement is about timing rather than about whether the Declaration matters.
Peacekeeping, and what the record shows
This is the longest stage by a distance — four books and 1,937 pages — and the one people most often read first and out of context. Shake hands with the devil goes first within it: Romeo Dallaire, the force commander in Rwanda, on the mission he was given, the warnings he sent, and the refusal to act on them. It is a primary source and a very hard book to read. Every subsequent argument about UN peacekeeping is conducted in its shadow.
A Problem from Hell is Samantha Power on American responses to genocide across the twentieth century — Armenia, the Holocaust, Cambodia, Iraq, Bosnia, Rwanda. It is not a book about the UN, and it is here because it establishes the pattern of state inaction that UN failure usually gets blamed for. Deliver us from evil surveys the 1990s missions from the inside with access to Boutros-Ghali and Annan; William Shawcross is sympathetic to the peacekeepers and unsparing about the gap between mandates and means.
Peacekeeping in the Midst of War is why this stage does not end in despair. Lisa Hultman, Jacob Kathman and Megan Shannon analyse mission-level data and find that peacekeeping with sufficient troops measurably reduces battlefield and civilian deaths. It is academic in style and it is the corrective to a literature naturally dominated by the catastrophes.
From inside the building
Chasing the Flame is Power's biography of Sergio Vieira de Mello, killed in the 2003 Baghdad bombing, and through him a history of what the UN actually does in the field: Cambodia, Bosnia, Kosovo, East Timor. The single best book here for understanding the organisation as a career and a set of impossible tradeoffs. Emergency Sex (and Other Desperate Measures) is the view from the bottom of the hierarchy — three junior field staff on Cambodia, Somalia, Rwanda, Haiti and Bosnia, written with a bluntness that prompted attempts to suppress it. End with The Best Intentions, James Traub on Kofi Annan's tenure at close range: Iraq, oil-for-food, the Responsibility to Protect, and a reform effort that mostly failed. It ends the path on the right question — what a secretary-general can do when the members do not want it done.
Fourteen books, about 5,370 pages. The stage sequence with goals and lengths is on the United Nations path.