It would stretch the definition of "diary" a bit, but I cannot recommend The Dept to Pleasure by John Lanchester enough. This book had me hanging on to every page and I just couldn't stop reading it once I began. The trouble is that trying to explain what the book is about without giving away the surprises is practically impossible. Let's just say it's about a man taking a journey around France while talking about food. But, this is not a book about food, even though there are recipes in the book haha. Like I said, I can't tell you WHY this book is so good without delving into spoilery territory. The best way to read this book is to going in completely unaware so try it and let me know what you think.

I have a good one for you. It's by Laurie Halse Anderson and it is called "Speak." The whole book is written in diary format by a girl named Melinda who basically loses the ability to talk to others around her after a traumatic incident. I cannot explain more about what the trauma is about without spoiling some elements, so you'll just have to take my word for it that this is a really good book. I found out much later that the author actually based some elements of the story on her own life, which is a bit sad, but explains why she was able to write everything in such a clear and expressive manner.

Decent suggestions overall, but for some real psychological horror in diary format you have to check out Diary by Chuck Palahniuk. I know his writing is not everyone's cup of tea, but Diary is one of his best books in my opinion. The author is a woman named Misty who writes in the diary while her husband is in a coma after a botched suicide attempt. To make things even worse for Misty, her husband worked as a builder, but his shenanigans at work has resulted in a ton of lawsuits, which his wife now has to deal with. Things only become weirder and weirder from there on out and even though the book is somewhat farfetched (like all horror novels to be honest), it managed to send a few chills up my spine.