For sure Mercury on the MC can mean a career in mercurial pursuits. Your dreams of being a successful writer is a tidy testament to that.Also, the ruler of MC Mars is located in Virgo, which is the domicile and exaltation of mercury. He also on the border of 2nd and 3rd house cusp. In your case it could mean making money (2nd house) from communications/publishing ( 3rd house).
Note that Mars is retrograde and this could mean that whatever he rules in the chart may be delayed or more positively could be 'introverted' in some manner. As your mercury comes under Mars' jurisdiction we see that you may be quite cutting/sharp/focused internally, in the realm of thoughts and the imagination. This is a good quality to have as a writer because you are then able to parse out the most pertinent pictures/ideas in your head which will then be used to form the basis of a coherent piece of work. Also Mars himself is under the rulership of Mercury who is the whole sign ruler of your 2nd house.
Exalted Ruler of your MC is the Sun and he is in a happy position in this chart. He is in Aries which he exalts in and is placed in the 9th house which is his joy. He being in the same sign as your MC may mean that you could be honoured in your career as the Sun and MC has parallels to the recognition and honours/awards. You having a Leo ascendant makes it all the important as then we see that you are the one who is the Sun in your chart. Things won't be so seamless because ol' saturn is there right beside him. Nevertheless, it is the Sun who dominates in the configuration.
Another plus for you earning in writing is the north node placed in your 3rd house. This might mean that when you pursue 3rd house matters you will gain in fortune.
So the indications of being a professional writer are there but there may be delays/ set-backs along the way. As to whether your book will be a success perhaps you could do a horary for that and ask the people in the horary section of this forum.
