How to approach automated text writing?
You should probably do some reading in the field of "Natural Language Generation", since this seems to relate most directly to your question. 

But the way you have described the process -- "text mining...text building" -- leads me to wonder if you are aiming for something much more ambitious.  It seems as though you aim to automate the process of 1) reading natural language texts, 2) understanding the meaning, and then 3) generate new texts based on that semantic knowledge.  I'm not aware of any general-purpose end-to-end systems that can do that, not even specialized systems by the likes of Palantir.  What you are aiming for would probably pass the Turing Test for fully capable Artificial Intelligence.