Hello
Finite volumes part of Telemac2d does not have an official theory document as it is the case for finite elements (the book of Jean Michel Hervouet). However, you can find a brief description of the framework as well as the FV fluxes implemented in the proceedings of the Telemac user conference of 2012 in Oxford. These proceedings can be found
here.
The paper dealing with FV is the one of R. Ata starting in page 93.
I hope this helps
with my best regards
Riadh Ata