  This paper focuses on the documentation work in the mathematics teaching. We
show up the individual and collective components of this documentation. We
present a new theoretical framework, the documentational approach which seems
adapted for studying this issue. We applied it on a particular project of the
French teachers association S\'esamath: the collective design of an online
mathematics textbook. We present a methodology for observing both this
collective project and the case of Ana\"is, a teacher involved in this project.
We study her professional interest in S\'esamath, her didactical interactions
with the other project members, and their effects on her professional
knowledge.
