If you are in solitude and have engaged your mind in reading, writing or other similar activities, that is still solitude. It is not the finest type though, it is more like pseudo solitude. The ultimate solitude is when you are aware of each passing moment, you are not dull and you are not sleepy, you are awake and alert, and at that, you do not feel restless; you do not feel the urge to always do something. You are at peace within. When you are face to face with your own mind, sharply looking at it directly, you are in solitude. A yogi who has mastered the art of living in solitude, without fail, will always be in solitude even amidst the greatest crowd. His quietude remains unaffected by the noise outside. His inner world stays insulated from the outer one.
In solitude, after initial periods of restlessness and stupor, bliss starts to flow through your very being. Everything becomes still. Your mind, senses, body, surroundings, flowing river, waterfalls, absolutely everything becomes still. Anahatnada, unstruck sound and beautiful other sounds start to manifest themselves. However, they can cause a deviation. A good meditator continues to stay disciplined and focused. Living in solitude requires great discipline. With self-discipline, you can achieve just about anything you can imagine. Disciplined living in solitude is tapas, an austerity, in its own right. It is the quickest way of self cleansing.
Reference : A Million Thoughts. Om Swami.