The Windrush Betrayal: Exposing The Hostile Environment by Amelia Gentleman.
This book will easily be in my top 5. Often heartbreaking; it's still such a necessary story to tell.
The author does a wonderful job of focusing on individual stories to highlight the awful failings in government policy.
Indeed the author, and Guardian journalist, was at the heart of changes being made (still a work in progress) for those of the Windrush generation affected by the nonsensical Home Office targets.