Over the years, I have observed that a properly written user's manual makes an excellent, if not the best, software requirements specification. I and several students have compiled a collection of case studies in which the production of user's manuals served as the requirements specification process for software developments. These are reported in
Berry, D.M., Daudjee, K., Dong, J., Fainchtein, I., Nelson, M.A., Nelson, T., Ou, L., ``User's Manual as a Requirements Specification'' PDF preprint

Three of the case studies involved significant pieces of software produced as master's thesis projects for me as client. One of the case studies involves a master's student adding to an industrial software development project with a non-documented requirements process his production of a user's manual as the project's documented requirements specification. In each case study in which the software is completed, the customer was very satisfied with the manual, the software, and the match between them. In each of the last academic and one industrial case studies, the time savings of having a fully documented requirements process is noted. We report on the events of each case studies and the lessons learned.