There are several synonyms for the phrase "softback book". One commonly used synonym is "paperback book", which refers to a book with a flexible cover made of paper or cardstock. Another synonym is "trade paperback", which is a larger-sized paperback book that is often printed on higher quality paper. "Mass market paperback" is another synonym that refers to a smaller-sized paperback book that is typically printed on cheaper paper and sold in large quantities. "Saddle-stitched" is a synonym that describes a small booklet-style softback book that is stapled together along the spine. Regardless of the synonym used, softback books are a convenient and affordable way to enjoy reading for many people.