You're welcome. I was looking for the online version of Wilkin's Essay, but it seems to have disappeared since I discovered it on the Web in the previous decade. There is another book on invented languages, that I have not read, but just found while looking at Amazon:

Arika Okrent. 2009. In the Land of Invented Languages: Esperanto Rock Stars, Klingon Poets, Loglan Lovers, and the Mad Dreamers Who Tried to Build A Perfect Language.

I skimmed the first chapter and it looks good. It's written by a linguist and has a light style.

Ceci n'est pas un seing.