I haven't actually read 'Two More Days' yet myself, but I did some digging since I've read almost everything else by Hoover. From what I understand, it's a novella in that multi-author 'The One More Day' charity collection, not a standalone novel. From summaries and spoiler threads, the story follows a woman named Augusta who makes a deal with a mysterious entity at a carnival booth, trading two days of her life for a glimpse at what her future would be like without her boyfriend.
The twist ending I saw discussed is that when she returns from the vision, she realizes the cost was real and she's now dying two days earlier than she should. The final scene apparently has her with her boyfriend, choosing to spend her last, unexpected moments with him, which gives the whole 'two more days' title a poignant, literal meaning. It seems like a classic Hoover blend of a love story with a heavy dose of tragedy, focusing on the value of time and love.