From the DLC's storepage
About: The Dropsy Warm Damp Hug includes the official Dropsy digital soundtrack by Chris Schlarb, the Eternal Hug music EP by Jay Tholen, and Dropsy's Book of Secrets - a collection of tips, sketches, concepts and ramblings from designer Jay Tholen.
So what I would expect would be: two soundtracks (.mp3 & .flac) as well as a document with hints, basic concept art and notes from the designer. (Hopefully) nothing you would need to enjoy the game to the fullest.*
I guess you got confused by GOG using screenshots from the game, rather than from the Dropsy's Book of Secrets artbook
* I've read there's a puzzle the developer wasn't proud of, but they left a solution... somewhere on the internet. There might be a link in this sub-forum, but I don't want to spoiler myself.
thanks for the info. I think they should have made that clear.