Is hardness of heart considered a sin for which its possessor will be held accountable? What are its manifestations, causes, and how can it be remedied? What are the books that have addressed this topic?

Hardness of heart varies in degrees. Some degrees lead to committing sins and abandoning obligations, for which the doer is blameworthy, while others are less severe. It is obligatory for a Muslim to strive to rid himself of it, because it is a reprehensible disease. Among its manifestations are: heedlessness of the remembrance of Allah, lack of humility, little shame and fear of Allah, indifference to committing prohibitions, abandoning good deeds, and lack of mercy towards creation.