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.

I would personally choose to use what is called contrastive focus reduplication (thank you, Vsauce, for that one) and call it a book book.