I'd like to start by mentioning that I'm a subscriber from pretty much the beginning and have purchased quite many books up until now.
Now with that said. I find those 20 something pages popping around at roughly the same price of a full book to be pretty offensive.
I'm never, ever buying them (I actually bought one at some point thinking I'd be getting the whole series since it spanned multiple chapters, but hey, I was wrong) and it would be nice to be able to simply hide them as stumbling upon them during browsing is a bit of an annoyance.
A rather easy way of going about it would be to allow users to flag tags that they want to be hidden from them (which could be a nice feature anyway on its own) and create a tag specifically for the releases that require additional purchase while not a book chapter.