The Tunnels of Cu Chi by Tom Mangold and John Penycate (1985) is the definitive account. Two British journalists spent years interviewing both American tunnel rats and Vietnamese tunnel fighters; the book gives equal weight to both sides and remains the most thorough explanation of how the tunnels worked and what it cost to fight in and around them. Read it before you go.
The Things They Carried by Tim O'Brien (1990) is fiction built from the author's own service in Vietnam. It is not about Cu Chi specifically, but it is the clearest literary account of what the war felt like from the American side — the physical weight of equipment, the moral weight of what was asked of young men, and the impossibility of making clean sense of any of it afterwards.
Kill Anything That Moves by Nick Turse (2013) is a documented history of American conduct in the war, assembled from military records, civilian testimonies, and investigative reporting. It is not comfortable reading. But it explains why the Cu Chi area was so heavily targeted, and why the people who built these tunnels built them.
The Mountains Sing by Nguyen Phan Que Mai (2020) is a Vietnamese novel that follows four generations of a Hanoi family through the French colonial period, the American War, and its aftermath. Written in English by a Vietnamese poet, it is the best introduction available in English to the war as experienced from the Vietnamese side — the land reform campaigns, the bombing, and the long reconstruction that followed.