So I have this audible subscription that lets me download one book a month for the same price. However it makes no discrimination between a little novella and a huge chunky tome and the latter and therefore more cost effective. I've really enjoyed downloading some chunky tomes in the past few months as well because I quite like getting lost in the sprawling maze of a big book. e.g.

-Perdido Street Station - China Mieville
-Under the Dome - Stephen King
-Titus Groan/Gormenghast - Mervyn Peake

What other doorsteps would people recommend?