Why people stop reading
Three reasons usually. The book is bad and they keep finishing it because they think they should. The book is good but slow and the moment for it has passed. They have not picked up a book in two months and the next attempt feels like work. All three problems have the same solution: a different book.
For people who used to love thrillers
Lee Child's Make Me. Andrew Gross's 15 Seconds. David Ellis's The Murder House. All three are plot-engine fiction engineered by writers who respect the reader. Make Me is the right starter if you have not read a Reacher; 15 Seconds is the right starter if you have read every Reacher already; The Murder House is the right starter if you want a Patterson-collaboration thriller with actual craft.
For people who want lighter reading
Janet Evanovich's Tricky Twenty-Two. Krista Davis's Diva Steals a Chocolate Kiss. Both are comic cozies that finish in a sitting and remind you that books can be fun. The Plum series rewards reading in order but you can start anywhere; the Diva series is similar.
For people who want adventure
Clive Cussler's The Chase. Isaac Bell, 1906 San Francisco, the chase of a serial bank robber. Pulp pacing, period detail, easy entry point. The 'Visit to the 1906 earthquake' chapter alone is worth the buy.
The fundamental advice
Pick something short, something fast, and something you do not feel guilty about. Reluctant readers who try to start their comeback with Anna Karenina almost never finish. Start with a Patterson collaboration, a Reacher, or a Phryne Fisher. The second book will be easier. The third will feel like reading.