I have a bias towards the slim ones. (I am talking about technical books here ;-). By slim I mean the books I can read lying on the couch. That would approx to the books with no more than 400 pages I guess. GoF's design patterns has more pages, though I consider it as slim. I think you got the idea.
Some of the best books I have read are the slim ones (Effective C++, Practices of an agile developer, etc.). They are cheap to buy, doesn't take too much real estate on my modest book shelf, takes less time to read, and usually I believe I learn more from them than from their heavy weight counterparts.
I agree there are some outstanding books out there which are huge (Code Complete for example). Those are the rare cases.
I am interested to know your favorite slim technical books.