Ruin of the Heart is a book in the Violet Area of the game, Ib. It is located in the upper-left corner of the small room where Red Eyes is found. In the True Guertena Exhibit, the book is right in front of the entrance to the Violet Room.
"If your spirit suffers too much, you will soon start to hallucinate..."
"And in the end, you will be destroyed."
"And more worrying yet..."
"Is that you will not even be conscious of that fact."
This book is in reference to the game's invisible Doom Counter. When Ib or Garry make unwise decisions throughout the cursed gallery (i.e., destroying exhibits, viewing Sinister Painting), they will begin to see hallucinations, such as hanging Ibs in the Sketchbook, or The Hanged Man's red eyes. These decisions will also increase the chances of obtaining a bad ending.
It should be noted that, in the Red Eyes room where this book is found, Ib and Garry have different perspectives on the room: Ib sees everything as adorable bunnies, while Garry sees them as scary blue dolls, implying that one of them is hallucinating. Since Garry is more affected by the gallery's scares, it is possible he is the one hallucinating. However, earlier, Garry noted how "this is even getting to her" after viewing the Couple painting, and followed by Ib's nightmare, it is definitely likely Ib is the one hallucinating.
Regarding the "in the end, you will be destroyed", this is in reference to the various endings available in the game. The more Ib or Garry's spirits "suffer" throughout the game, the worse the ending will become. This ranges from A Painting's Demise as the "worst" ending (with little Bond Points with Garry or Mary, almost all bad decisions), to Promise of Reunion as the best ending (Ib and Garry are alive and remember each other). Of course, throughout the game, the player will "not even be conscious" of the fact their decisions influence the ending.
- This is the only book that appears in the True Exhibit.