Recently I have been introduced to a couple of slang terms for the types of books I like. I was wondering if there were anymore similar descriptive slang for books like these.
Bubblegum- A fun little story (probably novella sized) that is really good but is over really quickly and you feel disappointed that it's over so quickly.
Popcorn- A long book or a long series that you binge on even though it isn't well written because it is more about quantity than quality and you just wanted something to snack on for a while.
I found these to be pretty good descriptors of books that express more about the experience you have reading the book than any levels of genre/subgenres.
The slang term is...
dead trees (techopedia.com)
Dead tree edition is a slang term used to describe any publication that comes in printed form, even when there is a paperless edition available.
I would personally choose to use what is called contrastive focus reduplication (thank you, Vsauce, for that one) and call it a book book.
Consider "birdcage liner". Typically used in reference to newspapers, but widely understood as printed material fit only for a bird to poop on.
As a well-read AE native speaker, I've never heard of "pap", so I can't recommend it for a general audience. I like "birdcage liner" - quite evocative - but honestly, the most direct expression is the one you said yourself:
This book turned out to be an unimaginative waste of paper.
That will be well understood and is fairly common. Other expressions include not worth the paper it's printed on - quite common, but doesn't slot neatly into your sentence. For a single word, drivel is probably your best bet - it's both harsh and fairly formal in register, so more likely to be seen in writing than garbage or trash, which are probably more common in speech. Drivel also has the advantage of usually being used to describe written works, though it doesn't strictly have to, whereas garbage is obviously much more general.