As far as I know, the book is out of print and available only in Russian. So, it is not easy for me to track down information about it. Some reviews online make it clear that it is a work of mathematical fiction in which Fermat's love of mathematics is relevant as he deals with "Three Musketeers"-style duels and fantastical challenges. We also know that it included a description of Fermat's Last Theorem, including the fact that it was (at the time the novel was written) famously an open problem in mathematics.

Thanks to Thomas Riepe for bringing this work of mathematical fiction to my attention and to both Oleg Smirnov and Vijay Fafat for helping to track down some information about it.