public String toString() {
        StringBuilder sb = new StringBuilder();

        sb.append("Title: " + title + "\n");
        sb.append("Author: " + author + "\n");
        sb.append("Release year: " + releaseYear + "\n");
        sb.append("Page count: " + pageCount + "\n");
        sb.append("Publisher: " + publisher + "\n");
        sb.append("Category: " + category + "\n");

        return sb.toString();
    }