If I hear the print run is going to super low or or the pre-order reaction will dictate the likelyhood that the book will reach print I will pre-order. For anything that would be small or personal press I prefer to get it directly from the creator so they get all the money for their work.
An example being, the Adam Hugh's Cover Run book I am not terribly worried about getting a copy of from Amazon or whatever, but the Spectrum book and pretty much anything from Underwood/Arnie and Cathy Fenner is an instant pre-order, they always deliver on content and quality. Their book also tend to sell out and then the price shoots through the roof if I ever want to pick them up later. I would include most bookos from Flesk Publications but I work for him so I usually get a comped a copy or two.
I love Sketch books there is nothing better then thumbing through your favorite artist's book and being so turned on by the creativity that you just want to draw and draw till your just have bloody numbs. In short yes per-order, totally.