The Midnight Library by Matt Haig

The Midnight Library. This book is a gem that makes you think about regrets in a novel way. The main character, Nora is depressed and makes a fateful decision to end her life and finds herself in the Midnight Library, an alternative universe where all her unlived lives are stored. It's a thought provoking read that sticks with you.  (Affiliate link alert. If you buy from this link, I earn a few pennies for this recommendation and you get a great book.)