I don't order books online often. When I do, they're typically gifts for others.
It's not that I don't read dead trees, I just prefer to browse in a bricks-and-morter store, even though I usually know what I'm looking for and don't need to review before purchasing. Fond memories of childhood hours spent in the neighborhood library doubtless contribute to this behavior quirk.
But I seem to be developing a habit of buying books-to-be on the web. It started in 2005 when I pre-ordered Huw "Lem" Davis's Book of Random; even though it took forever to materialize, I've been tickled pink to interact with bunny on physical pages.
Most recently, I've taken an opportunity to support the fascinating team at Damn Interesting by pre-purchasing "the book". It will certainly exceed the digital edition's paper-weighing capabilities, but they've mostly enticed me by establishing a relationship by giving away great content, that they can now monetize via promises of a fleshed-out compendium in a pleasant-to-browse format.
I already pay for John Gruber's Linked List and I'm sure someday I'll be paying for a movie from Ze Frank. I've already bought T-shirts from each (and from Calico Monkey)… and in every case I feel like I'm bartering with a friend, not just purchasing product. This is clearly the future of publishing.