Hello, Its hard to pinpoint which book that is. Observing the book's appearance: its leather-bound, decorated bound in marble pattern, and contents are full of illustration of historical fashion. I don't think its an easy access kind of book (it isn't openly shared through the internet and might possibly only exist physically). I tried looking for books that align to my observation. I recommend you searching through keywords like 'Regency Period' if you'll be persistent.
Following this thread to get information as well.

Uhh this might be an odd request but does anyone know what title of the book the author is looking at in the afterword for the dresses ideas?