There are few texts written about software documentation as a subject unto itself. However, since all software development, testing, and maintenance processes depend on the requirements, you can start with the following texts:
R. Dorfman et al. (eds). Software Requirements Engineering, Second Edition, Los Alamitos, CA: IEEE Computer Society Press, 1997.
Alan M. Davis, Software Requirements: Analysis and Specification, Englewood Cliffs, NJ: Prentice Hall, 1990.