LaTeX is a good document preparation system. It is the great choice for researchers for scientific documentation especially those with lots of mathematical equations. This is a free software and the project site gives all details. The following is a popular book to get started.
Bibby, D., Lamport, L. (1994). LATEX: a document preparation system : user’s guide and reference manual. Germany: Addison-Wesley.
Dr. Selvaraj Vadivelu 