As mentioned, there is one book on Paris by prof Li Heng from China, some pictures from it can be found here:
http://www.goldsword.com/sfarmer/Trillium/liheng.htmlHowever, it is written in Chinese. After the publication of this book in 1998, her assistent Ji Yunheng has done phylogenetic work on the genus which was published in the article: "Phylogeny and Classification of Paris (Melanthiaceae) Inferred from DNA Sequence Data" in Ann Bot. 2006 July; 98(1): 245–256.
As the genus is most diversified in China the treatment in the FoC would be about the only key one can use and it was compiled by Li Heng and Ji Yunheng so would represent the latest ideas about the genus from China:
http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=124055One could question however the validity of such a treatment if a species is split into 8 varieties (P. polyphylla)....
There is no book written by any botanist from the West as far as I know that could respresent a western approach to taxonomy for Paris so we are stuck with a lot of splitting and endemic species.