I'd go for Freidrich Engels' "The Condition of the Working Class in England".
It's an absolute masterpiece and gives an account of how people in England lived during the Industrial Revolution. A large part of the account is based around his time in Manchester and the conditions people suffered were absolutely horrific. Beyond anything you can imagine. It's also one of the early books that studied society 'scientifically'.
I love the way it talks about Deansgate, Oxford Rd and Angel Meadow and it's almost impossible to imagine how life must've been then. It's almost unbelievable. The accounts of how miners and their families were treated in Northumberland is horrendous.
It's a shocking but brilliant read and is one of the most influential political books ever written.
A masterpiece.
A notable mention for the "Road to Wigan Pier" by Orwell too which is in a similar vein.