For me, a book becomes unforgettable and worth recommending when it has well-developed characters that I can connect with on an emotional level. When the plot is engaging and keeps me turning the pages, I know it's a book I will remember long after I've finished it.
Another important factor for me is the writing style of the author. If the prose is beautiful, engaging, and immersive, it adds a whole new layer of enjoyment to the reading experience. A book that stays with me is usually one where I can vividly imagine the scenes and feel like I'm a part of the story.
Furthermore, a book that tackles complex themes and leaves me thinking long after I've finished reading is one that I'm likely to recommend to others. Whether it's a philosophical exploration, a social commentary, or a poignant reflection on human nature, I appreciate books that make me reflect on deeper aspects of life.
Lastly, a book that surprises me with unexpected twists and turns, or with a unique and original storyline, is one that I won't easily forget. When a book keeps me guessing and shocks me with its revelations, it leaves a lasting impression and I'm more likely to rave about it to my friends and family.