There appear to be two different books/works called 'The Book of Masks' combined on this work page. Most of them are copies of the Atlas Arkhive publication entitled 'The Book of Masks: An Anthology of French Symbolist & Decadent Writing' -- this is not by Gourmont but is intead an edited collection of pieces by various writers, including a few short pieces by Gourmont. The other work (from which the anthology borrowed its title) is a 1921 collection of critical essays by Gourmont on various Symbolist and Decadent writers.

There are at least a few of these in the editions list here.

Ideally these two books would have separate work pages but there are probably a few copies here which can't be identified as one or the other.