I feel like there's two common themes left by many negative reviews for this one, so before I say anything else:

1) This book is a work of fiction and despite it involving themes of depression and suicide, this is by no means a self-help guide for people feeling depressed. Neither should one assume the protag's feelings or thoughts on the matter are representative for everyone who has ever felt depressed as depression is such a personal and varied thing for people.

2) It's called The Midnight Library and the library serves as a plot element as well as a metaphor for one's internal state of mind. Majority of the story does NOT take place in a library and the books are also a metaphor for "what if" scenarios in her life. This is a novel more akin to a drama-based, non-thriller take on the Butterfly Effect, NOT a novel about reading or literature (did they even read the synposis?).

Points aside, I found the concept interesting and the progression of the plot and internal thoughts of the protag be natural and well-paced. Yes some of the "what if" scenarios are predictable and yes, maybe you can predict how the story will end by around the half-way mark in. However there were some plot elements that I didn't expect (spoiler free but relating to one of the mechanisms involving going to and from each different life scenario) and there are occasional low-key hilarious moments that nicely break up the more heavy themes and moments. I thoroughly enjoyed this and thought the ending was very well-executed despite being familiar (I've been on a Matt Haig reading marathon and I'm seeing a trend of strong endings in his novels).